Short version: Apple appears to have used estimates of economic benefits that favour itself, but which aren't outside the plausible range. Taken with its "created or supported" claim that makes it not too inaccurate, as marketing releases go.
There are a number of articles analyzing this statement by Apple. Three good ones are:
The consensus from the articles is that Apple isn't entirely responsible for 500,000 jobs. It's an economic calculation, with some standard, but disputable, factors thrown in.
A number of companies, including Microsoft, have commissioned similar research aiming to tally up such indirect employment by suppliers and other partners. The use of “job multipliers” has become common practice, sometimes by businesses lobbying for tax breaks from local and state governments.But the calculations of such multipliers are often fiercely debated both in economic and political terms. For instance, the Congressional Budget Office, which calculated the impact of the 2009 federal stimulus on jobs, has set estimates that vary between as few as 1.6 million jobs and as many as 8.4 million jobs.
However, Apple isn't actually claiming that all those jobs would vanish without it. It specifically says "created or supported" that many jobs. "Supported" means the job wouldn't go away of Apple were to vanish tomorrow.