After a little more digging I found some very cool answers in the docs: https://antithesis.com/docs/

This seems like a cool company and I don't want to nitpick too much, but gamers have no respect for history:

  Castlevania... [so] called because it is a Metroidvania game set in a Castle.

Ouch - this is precisely backwards. Metroidvanias are named after Metroid and Castlevania because those series practically defined the genre.

Also a bit frustrating because the first Castlevania itself isn't actually a metroidvania, it's a more conventional action-platformer. Castlevania II has non-linear exploration, lots of items to collect, and puzzle-solving, all like Metroid. So it's not too surprising Antithesis had to do a lot of work for adapting their system to Metroid - but I wonder if this work means it now can handle Castlevania II without much extra development.