Given that both have a history here, the big question is who benefits. I can think of a very likely motive for an internet vigilante to try to smear United Healthcare by making them look even worse. I can't think of a great motive for United Healthcare to try (and completely predictably fail) to suppress the story.
It is entirely believable that UHC is working with some crisis management firm to try to manage public perception, and that part of their strategy is to try to get media portraying Mangione positively taken off the internet, whether or not they have legal grounds to do so.
There have been so many past stories about companies using illegitimate DMCA claims that I'm surprised you consider it obviously not UHC or someone working with them. It is possible, but I don't think it's a forgone conclusion.
That said, I'd argue even if these are falsified reports (I don't personally have a strong opinion on this - I could see this going either way) it seems very much of interest to this community.
It's also interesting - it doesn't seem like there can be copyright on security camera footage (just like there isn't/shouldn't be on AI output) so even if the claim were brought by a different party it seems questionable.