So me running my car matters. I'm directly putting CO2 in the air, even if my contribution is small.
By "are a subset of a broader collective action" I mean as follows: if all members of a given class/collective stopped doing an act, would there be an impact? If all voters stopped voting, or if all families stopped flying, or if all grocery shoppers stopped buying meat, then yes, elections would break, and flights would decline, and meat would stop being sold. Even though my vote doesn't matter, and my family's ticket doesn't matter, and no, Purdue isn't going to stop factory farming chicked because I went vegan.
In Vitalik's case--and I don't mean him specifically, but using him as a stand-in for a broader class of questions--, two things are true:
1. There's no direct result between him getting on a plane and the flight existing. His impact on flight demand isn't small, it's zero, because flights existing are non-linear. Those flights were going to be there anyway.
2. He is sui-generis. He's not a vacationing family or a a travelling salesman. There's no collective problem of "man, excentric genius crypto billionaires fly commercial too much." That just isn't a problem. If Vitalik, or all Vitaliks stopped flying, nothing would actually change. Whereas if all families stopped flying, or all business travellers—or whatever other class the vast, vast majority of flyers belong to—stopped travelling, then yes, there would be an impact.
I guess you could say "ha! 'excentric genius crypto billionaires' isn't a collective! the right collective VB belongs to is 'people', and if all people stopped flying then there would certainly be an impact!". And that's certainly an argument, but I think both sides are arguable.
And lastly, I'm not actually arguing that he should fly: I'm asking 'what do you call this logical problem, where we want to condemn someone for doing a thing that doesn't actually matter at all except symbolically?
(Sorry for the ninja edits)