alt Hacker NewsCharlie 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.
I think it is a reference to the song by the Damned, but given Stross has lived in central Scotland for decades, you'd think he'd realise by now that it sounds more like someone from the Orange Order (Protestant supremacists) in these parts. There is historically a major issue with the Orange Order and Loyalists in the areas in and around Edinburgh and Glasgow, and they organise anti-Catholic marches in many working class communities as well as violence at football matches. I have met him and doubt this is what he is after, but it comes over like this.