I think this is their attempt to EEE (Embrace, Extend, Extinguish) the Fediverse, so I'm strongly in favour of immediately defederating corporate instances as soon as they are created.
Given how federation works (and my limited understanding of it) I believe not much will change, since they simply can't Extinguish. It's all federated into different instances under different moderation teams. They hold no 'ultimate control' over anything beyond their own instances
I also have limited understanding, but these are huge corporations with huge userbases. If they start giving "helpful" input on how the ActivityPub protocol should develop, it may exert a strong pressure/influence due to their sheer size.
I know it's not the same situation, but it reminds me of how Google keeps trying to push shady stuff into Chromium, even though it's supposed to be free, open-source software.
I've heard about Threads, which frankly I think will never pan out (they have no particular reason to encroach on this "market" given how small and usually anti-corporate it is here - I think they only mentioned ActivityPub as a buzzword), but Tumblr?
I think you're right about the Threads situation. As always zuck is pumping out buzzwords and hype.
Tumblr announced they will be starting to work on activitypub integration in November 2022 but I guess either it's taking a long time due to how Tumblr works or they completely forgot about it
I'm not sure if Tumblr is going to actually do it, they have been working on it for a while. Automatic, the owners of Tumblr, did add it to wordpress already and they have open-sourced parts of Tumblr, so it would probably be fine