Why would he say that knowing that his superiors would know if he was undercover to infiltrate the group? It makes no sense for him to say he was on the ride and didn't do anything in order to preserve his cover if he was NOT on assignment to infiltrate the group because if it wasn't his assignment then he doesn't have any NEED to preserve his cover. Where do you think their assignments come from? His superiors. They're the ones that assign to him his undercover jobs and of course would know whether or not he was assigned to infiltrate this group. He wasn't trying to cover up the fact that he was on the ride, he was trying to cover up what he DID on the ride. Obviously he wasn't trying to cover up the fact that he was on the ride since HE voluntarily went to THEM to say that while he was on the ride he did nothing to try to stop the bikers from beating on the SUV and Mr. Lien to try to preserve his cover. It was his "did nothing" claim that was the lie.
His attorneys never made the claim that he wasn't on the ride. Why on earth would they when he voluntarily went to his superiors who already knew he was on the ride and why. He NEVER tried to cover up that he was on the ride and neither did his attorneys. They already knew he was on the ride, and that was never the problem... and it wasn't a problem because being on the ride was his assignment. The problem was why did he do nothing to try to stop what the bikers were doing in beating on the SUV and Mr. Lien, and his claim as to why he did nothing was to try preserving his cover. But then they found out from other video that he lied about doing nothing as he's seen in that other video beating on the SUV's back window. THAT was what he was trying to cover up, and THAT is why he was arrested... for lying to his superiors in saying he did nothing to try to stop the carnage in order to preserve his cover and their discovering that he DID take part in that carnage.
Like I said, it's not rocket science.