I have no doubts about Meta's intentions but what about blocking GMail?
No email provider prevents you from interacting with GMail users, of course. But at least GMail being based on SMTP let us use our own email provider (to some extent, GMail sometimes blocks hobbist servers) with both GMail and non-GMail contacts.
If a person on Facebook/Instagram/Twitter notices that they can move to a FOSS instance without loosing their contacts maybe they will do so, reducing the data Meta & Co can gather (like what you are looking at while scrolling timelines).
This is why I think Meta & Co will never adopt ActivityPub, because they would make it too easy for people to move away.