I don't believe they will stick with the ActivityPub outside of their initial start up period for Threads. While they are federating though it will be a fantastic opportunity to introduce users to Lemmy from Threads. If the Lemmy integration in Threads is anything like Mastodon's then I think it's more then worthwhile given that Lemmy is offering something Threads isn't currently covering.

However I think the onus should be on Meta to Federate respectfully, if they aren't willing to federate without contracts getting involved then I think that's completely against the spirt of the Fediverse and Meta shouldn't be allowed to operate in that capacity.