Charlie explains why his site is named this, but the word "Antipope" does mean something already and it means specifically a person who says they are the Pope, but you recognise somebody else as Pope instead.

It's thus sometimes unclear which people are "really" Antipopes until some time passes and you're like OK, Steve was an Antipope, the Roman Catholic Church still exists, the Pope is now this Dave guy, so his predecessor Geoff was also Pope, and Geoff's rival Steve's group died out, so Steve was an Antipope, even though for a few years my church said Steve was Pope, I now know that was wrong because God wouldn't let the One True Church vanish and once Steve died his whole church went back to Geoff.

Which is a terrible assumption given an antipope is just a self-declared pope

Charles Stross has lived in Edinburgh for years and you would think by now that something like "Antipope" comes off as Protestant supremacist rather than secularist in central Scotland. One of his close friends has a similar background and she had antipope as part of her email back in the nineties when I was acquainted with her... It caused a lot of issues when she was emailing someone who was Catholic, because they thought it was some kind of Orange/loyalist account.