I think it's fair for the alt-righters that put this convention together to say they were excluded. Probably fair to exclude them too, but imo making their own con in reaction to being excluded seems like a reasonable decision. the rage bait stuff is lame though.

Were they actually excluded, or did they feel excluded because they didn't like the new philosophy? I'm not sure if they were actual rules actually excluding them.

There weren't actual rules excluding them.

Compare the "about" page for the more mainstream Worldcon with the one for BasedCon:

https://www.worldcon.org/about-worldcon/

https://www.basedcon.com/?page_id=85

BasedCon's "about" has hot takes on guns, socialism, abortion, etc. Worldcon's "about" doesn't say anything about those topics at all.

Here's the Registration FAQ for next year's Worldcon in Glasgow:

https://glasgow2024.org/get-involved/registration-faq/

There are no hot-button American political topics mentioned either way there.

The last time I was at a SF convention there was a wide variety of political views among attendees, just as there have been among award-winning SF authors. BasedCon seems unusual not in what sort of views it permits but in what it explicitly rejects.

I would think that they excluded themselves because of what they perceived as too much inclusion, which is ironic in a weird way.