There's a big difference between demanding support while being an asshole and advising that obviously troll emails can be tossed in the trash without ruining your day.
James is totally free to do whatever he wants, of course.
That person is still demanding support - they're clearly using this library [minor edit due to garbage grammar] - they're just hiding who they are because they know their behavior is BS. Receiving mail like this has a mental and emotional toll even if it can be "tossed in the trash" you still have to at least start reading it to determine that it's a troll.
So this maintainer now has to read these messages, even if it's just to send them to the trash, and knows that the person sending this is a person that is using the library, and so any more work they do is helping this person.
Exuma is saying "you can only control your own reaction", which is pretty dismissive and contextually implies that shutting down the project is not a reasonable reaction. i.e. if you maintain a project, it's over reacting if you choose to stop maintaining that project just because you're being subjected to abuse.
I'm not sure how else to interpret Exuma's comment.
Something mentioned in the email message is private members of an interface. If they do abandon the project, then I suppose that whoever wrote that can make them public safely since there is not a new version, it is unlikely to break compatibility with newer versions if there aren't any. So, maybe abandoning this project, does possibly help them.
(This is not any kind of excuse for writing such a bad message nor a reason for continuing to maintain the project; it is just an observation.)
So, abandoning or not abandoning it isn't considering if it helps them or not. Rather, what is important to consider is how much it harms everyone else; how much it helps the person sending that message isn't important.