Links to source articles below.
Yesterday 30 million users signed up for threads, which is already more than active users in the fediverse.
Furthermore, it seems that Meta hasn't launched threads in the EU due to uncertainty regarding the Digital Markets Act. It is entirely possible that their intent to federate with other Activitypub instances is entirely a cheap way to avoid being labeled a gatekeeper and avoid other regulatory requirements or restrictions.
It's future use of ActivityPub to get better publicity or scrape a bit more data might be an added benefit but not it's true purpose.
We'll see if launch in the EU goes hand in hand with them turning on Federation. I suspect that ActivityPub and the Fediverse are merely an afterthought to them and a convenient way to avoid being impacted by certain regulations.
Edit: Found a brief overview of the DMA. Among other things they say:
"The DMA aims to ensure the interoperability of messaging services allowing users on services like WhatsApp to send messages to users on smaller services like Signal"
https://ec.europa.eu/commission/presscorner/detail/en/ip_22_6423
And open them up to anti-conpetitive and monopoly law suits? Meta has already been getting calls to be broken up for having the top two social medias, if they get the full trifecta then they're gonna have a tough time selling defederation to the FTC or E.U. Better for them to take 90% of the pie and allow others to fight over the scraps then try for full 100% and risk litigation.