I don't think it's "petty" (to use dang's wording) because the underlying issue ("cheating" to get a vaccine) is petty. You're right - the underlying issue is serious.

It's "petty" because the discussion isn't about the underlying issue, the discussions is about someone claiming that they were kicked out of YC for posting to Twitter about someone else posting about skipping the line to a separate internal forum. That's very far removed from the underlying issue of vaccines, and brings in so many other topics into play, many of which are just not best reasoned about in this kind of forum (e.g. is the tweeter telling the truth? If they are, was it ok that they publicly tweeted about something from an internal forum? Did they first approach the original posted? etc.)

Much as you or I might like the discussion to be "Does YC condone lying/cheating to get the vaccine early", that's just not the topic we can actually answer here, since we have no access to the real underlying facts (and probably only one side speaking).

That is what to my mind makes this issue "petty", not the seriousness of the subject matter.