This doesn't mean anything since "who they say they are" is an anonymous username with no real life correlation. Might as well be completely anonymous.
> that other people (the distro maintainers) believe that author to be the correct entity
No? Anyone can make an account and upload to AUR and it has exactly 0% to do with the distro maintainers. Packages can be removed if they're malicious, but websites can also be removed via browser-controlled blacklists (which I don't like btw but it's how it works nowadays).
> And any such compromise would, by definition, affect all users of the repo and presumably be detected by them and not by you in the overwhelmingly common case.
This is true of a popular website that advertises install instructions using curl | bash as well.
I've been using Linux for the past 2 decades and my general experience is that it is in no way more secure than Windows or Mac, just way less popular and with a more tech savvy userbase.