It's not about assuming anything or the fact that he's a vlogger or anything else. It's about general human decency. You don't accuse someone of something without evidence to back it up (and I know that's not you, I'm more referring to the OP). Tim described his story, gave the timeline, everything else in the video. Unless someone has a reason to believe he's lying (based on facts, not just your personal feelings about him), then shut up and let people live their lives.
Honestly, the discourse in this culture has gotten so toxic - people are just aching to take someone down - that they'll make up stuff about people. I try to live my life from a point of view that everyone deserves respect and the benefit of the doubt until they show me that they don't deserve the benefit of the doubt. You can disagree with people on lots of things, but do it respectfully and without ever getting personal.
Now, taking his story at face value - you could think that it's still irresponsible to go to the parks, but at the same time, by his account he's following the science, the guidelines laid out by the CDC in regards to quaranteeining, etc. And we know from the science that if you're fully vaxxed, the transmission period is very short compared to unvaxxed.
So criticize Tim all you want for his content, his approach, his style, etc. That's all well and good and fair game. But to make up things about him without any evidence to back it up is something that no human being deserves, regardless of whether you personally like them or not.