I think the argument is less "noone but power users would need this" and more "only power users will know how to apply 'Invert Selection' to get the behaviour they want".

ie there's an inductive leap between "I want to delete all files except this one" and "I can select that one file and then invert selection before deleting". At minimum it requires you to know what "invert selection" means. Ordinary users may take a different, more fiddly, path.

I don't know if that's the case here. My intuition says it is. However, in my optimistic heart I hope Microsoft did a bunch of usability testing and determined that ordinary users do grasp an "invert selection" concept more often than not.

That's how design for mass-use user interfaces works, right? By evidence-driven testing, not by what the programmer/designer thinks makes sense? Clings to idealistic belief.