It's unlikely that Apple's support for USB had any significant impact on the adoption of the standard.
There are two events that, in combination, are far, far more likely to have "jump started" USB adoption.
The first was the release of Windows 98 in June 1998. Prior to Windows 98, USB support in Windows was flaky at best. Support was hastily added to Windows 95 through a patch which was pretty universally panned at the time. Windows 98 added extra functionality and fixed many bugs.
The second was the release of the USB 1.1 Specification in August 1998 (though peripheral manufacturers would have had access to draft revisions much earlier than that in order to have 1.1 devices ready for market). The USB 1.1 spec had "Updates to all chapters to fix problems identified".
Texas Instruments' assessment of the adoption of USB:
The release of the USB 1.1 specification combined with the native operating system support offered by Microsoft enabled the rapid adoption of USB hosts in the PC. It also drove the conversion of many peripheral devices from legacy interfaces such as serial (RS- 232), PS-2 (mice and keyboards), and parallel ports (Centronix and IEEE-1284 for printers) to this common interface standard.
Published just after Windows 98, this article does a decent job at summarising the benefits the new OS would bring to peripheral makers:
Peripherals to surge with Win 98
Peripheral vendors--companies that specialize in modems, digital cameras, add-in cards, and the like--will likely experience an upswing in business following Microsoft's Windows 98 rollout, since the new operating system will bring built-in support for a number of emerging hardware technologies.
By far the most practical addition to Windows 98 is built-in support for the universal serial bus (USB) connector....
"Up until now, there has been no reason to implement [USB] on low-end products because it adds to cost," Bursley noted. "That said, we expect within the next year some inkjet printers will offer USB connections. Many already offer USB printers in Japan," she added.
But peripherals manufacturers are working on USB devices as PCs, toward the day when Windows 98 and USB connectors become more common. Eventually, economies of scale will help persuade companies to come out with more USB products.
"There will be a pretty impressive showing of peripherals this summer," said Rob Bennett, group product manager for Windows 98. "There are 250 devices due to be launched around Windows 98 and 100 in development [to be] released in the next year," he said.
John W. Koon's book USB: Peripheral Design summarised the landscape for USB peripheral manufacturers in 1998:
According to Dataquest and Intel's projectsion (USB conference, July 1996), USB PC shipments were estimated at 20 million units in 1997 and 100 million units in 1999. In addition, the Bishop Report stated that the USB connector market would hit $400 million in 1999. The estimated ratio of peripheral use per host PC is four to one.
Mid 1998 saw an explosion of USB devices onto the market. According to the USB implementers forum chair at the time, Stephen Walley:
Three main factors are credited for the large volume of development activity, according to Whalley. These include: USB becoming mainstream on all consumer desktop and most notebook PCs, the wide availability of building blocks for developing products (such as silicon and development tools), and the upcoming availability of Windows '98.
What About Apple?
Apple's release of the iMac spurred the recovery of the company, but it's difficult to argue that a single computer model (accounting for less than 5% of private desktop computer sales at its peak) influenced the adoption of USB in any great way.
In 1997, Steve Jobs had recently been re-hired as Apple CEO and was trying to put the company back on the path to success. He slashed their product line, penned a deal with Microsoft to get Office on the Mac for at least 5 years, and was pushing both firewire and USB as standards. USB at the time didn't support high speeds - the aim was for Apple to be interoperable with the world of PC peripherals that had largely had not been interoperable with Macs.
The earliest public hint at Apple support for USB was in October 1997:
Jobs' keynote, meanwhile, offered many promises and few details for the bedraggled company's recovery. Mentioning a 14-step plan, Jobs said Mac users should have faith in Apple's new board of directors, which includes industry insiders such as Oracle Chair Larry Ellison and Intuit CEO Bill Campbell.
He reiterated the company's pledge to pare down its product line and focus on "the 30 percent of our product line [that is] fantastic."
Apple will add some new products to its line next year, Jobs said, although he offered no specifics. He also said that in 1998 the company will support FireWire, Apple's high-speed technology for linking Macintoshes with devices such as printers and cameras, and Universal Serial Bus, which links peripherals like scanners and monitors.
However, prior to his announcement that Apple would likely support USB in 1998, the following had been ocurring:
- Jan 1996 - USB Specification 1.0 released, authored by Compaq, Intel, Microsoft and NEC, see also the full USB 1.1 Specification (PDF)
- Jan 1997 - Microsoft, in addition to previously helping to develop the USB standard, releases an updated version of Windows 95 enabling the use of USB devices
- Feb 1997 - Kodak demonstrate the first Digital Camera with USB connection for power and data
- June 1997 - Xirlink demostrated a USB Webcam
- June 1997 - Philips demonstrated a USB hub enabled monitor, USB infrared port (for already existing wireless mice and keyboards) and USB speakers
- June 1997 - Thrustmaster demonstrated a USB Joystick
- June 1997 - Sony began producing laptops, these had both USB and Firewire ports as standard
- July 1997 - HP announced future computers would be USB enabled, and showed off PCs with USB keyboards which included multimedia buttons, as well as USB printers and scanners
- July 1997 - Canon had functional USB Printers available for testing
- July 1997 - Dell, Gateway, NEC, and Panasonic were selling consumer PCs with USB ports
- Oct 1997 - Connectix (now Logitech) QuickClip USB Webcam available for purchase for $99
- Nov 1997 - Connectix (now Logitech) QuickCam VC USB Webcam available for purchase. Over 1 million Connectix units sold before the release of Win98. This camera didn't work with the iMac until September 1998
- 1997 (Month unknown) - USB-IF membership increased to more than 400. Over 500 products were in development worldwide. First third party USB developers' conference held.
Essentially, all of the major technology players had decided to support USB as a standard and peripheral makers were switching over. Prior to the release of the first Apple with USB support, USB devices and computers with USB ports (for example: the Dell GXa, one of the best selling business PCs of the era, IBM's PC 300 Series) were both widespread.
There were well over 50 USB peripherals on the market prior to the launch of the iMac in 1998, and hundreds more came onto the market throughout that year
Retail computer sales in November 1998. Over 84% are PCs from the top 4 brands at the time, with Apple coming in at under 5%. This doesn't even take into account business PC sales, which were likely to be more heavily lopsided in the PCs favour.
When you consider that Apple accounted for less than 5% of the desktop OS share at the time, and the fact that all major PC manufacturers were including USB ports on their computers, and that peripheral manufacturers were releasing either USB-only or dual-connection devices, it puts the claim into the realm of the extraordinary, and I've seen no extraordinary evidence backing it up.