I think the simpler answer is more likely to be correct. The Fediverse isn't big enough to really bother Meta, but ActivityPub is a convenient way to seem cool, so they'll partially support it as long as it doesn't cost them all that much. Once the marketing gimmick has run it's course, they'll drop it.

I think the same was true for XMPP. I don't think they planned to kill XMPP and I don't think they plan to kill ActivityPub. But they did kill XMPP, and they'll probably kill ActivityPub by accident as well when they support it just well enough to pull people over.

So I'm not worried about some Meta conspiracy to kill ActivityPub, I'm worried about getting steamrolled on accident for a similar reason that people don't want to share locations of where they took pictures: they don't want the big mass of people coming to destroy something unique.

So my recommendation is to push for making everything E2E encrypted by default, and have every message cryptographically signed by the contributor. If there's something ad companies hate it's privacy, and that's what we should be pursuing. I'm not sure how that works for Lemmy, but surely there's a way for instances to manage who can decrypt messages.