I think the critical difference is “Meta pushes for changes” vs “Meta pushes for changes with the support of thousands/millions of users”.
If Meta convinces Thread users that a certain change is good for them, it’s going to be that much harder for the people developing ActivityPub to push back on those changes. And even if the developers succeed, Meta can just use that to say “fine, we’ll fork off and make our own ActivityPub with data collection and advertisements” and if enough instances in the Fediverse are reliant one Threads for engagement they may just switch to the Meta version of ActivityPub, taking a chunk of our community with them.
And maybe that’s alright for some folks, but a lot of us don’t want any of that to happen, even potentially. I think it’s pretty unethical to deliver people into the maw of the beast like that, so to speak.