@atomicpoet
the answer might be much simpler: https://reb00ted.org/tech/20230625-meta-why-activitypub/

" 3. The European Union is coming down hard on Meta, demanding all sorts of interoperability as part of its Digital Services Act. By implementing a bona-fide W3C interoperability standard as part of a new app, Meta can signal both cooperation with the EU authorities, while delaying opening its core business as long as possible.
[..]
they have to throw the regulators a bone, on occasion."

(by @J12t )