Groups like Extinction Rebellion aren't exactly hurting for press coverage due to the nature of their protests. I think Evans thought he could bring Tibor, tell them he was a "filmmaker" and they'd be glad he was on-board... They want participants, not some guy with a camera who would be the first to yell "Press" the minute the police came to haul them away.
Also, another thing is that typically these groups have a chain of command and someone giving approvals or denials to the press. My bet is he showed up and got told he had to email their comms director for approval, that they wouldn't let him film unless he did that first. He probably tried to wow them with his YouTube numbers hoping he could convince them but the didn't budge. If he had to admit that his filming was halted because he didn't do the most basic first step he looks extremely foolish.