That’s almost exactly what I was thinking before listening to the podcast.
But there she explained how ActivityPub was missing some of the feature they wanted because of its instance-centric approach and how trying to change that would have been hard (given how sceptical towards changes and everything corporate-related the fediverse community can be), and so they opted for a new protocol since the goals of the two project were with different aims.
Still not 100% convinced tbh, but I can’t deny she has a point…