Yes, he was. He admitted it in a video on his own Twitter, and even posted videos he shot at the location. (The latter he also posted on youtube; in one of these he says he's "anti-government".)
And Fox News and CNN covered his participation too. I can only find a transcript from the former though:
An anti-Trump activist who once said he wanted to "rip" the president out of office entered the Capitol Building Wednesday alongside a mob of pro-Trump protesters, but he said he was just there to "document" it.
And some secondary coverage of the latter (CNN) coverage on CBS58:
The US Capitol Police employee who shot Babbitt has been placed on administrative leave and their police powers have been suspended pending a joint Metro Police and USCP investigation, USCP said. [...]
Two people who saw the shooting, left-wing activist John Sullivan and documentary filmmaker Jade Sacker, provided CNN with video of the incident and described the moments beforehand.
He was now arrested and charged with entering a restricted area, interfering with law enforcement, and disorderly conduct, in part because he did not have any press credentials, but also because he was heard saying things like “We gotta get this shit burned.” (Press coverage; FBI Affidavit.)