I don't think Facebook really cares about attracting the current Mastodon/Fediverse users to their new social network. I think they have chosen ActivityPub for two reasons:

1. They want a product to compete against Twitter. And with Musk rapid enshittificating it, they need to act quick. ActivityPub is open source and proven to work, so Facebook has less work to do, and can release their new product sooner.

2. The main downfall of Twitter is advertisers leaving the site because they don't want their ads next to hateful comments that are now allowed under the new management. But this is a problem that can be neatly solved with defederation! Each advertiser can have their own instance (instance-as-a-service provided by Facebook), and they have more granularity in decided in which parts of the network they want to participate, and which parts to defederate. Sure, we the original denizens of the Fediverse will defederate from anything Meta, specially from ad instances. But most of the users of this new Meta-fediverse will remain in their Meta-approved instances, oblivious to the world outside them.