I swear, I'm so tired of naive takes about "good" and "bad" corporations.
Corporations are corporations. They are groups of people legally mandated to make money for their shareholders. They're not individuals.
So yeah, I'm fairly confident that them taking steps towards joining ActivityPub is some mix of high ranking people thnking interoperability is cool, some other high ranking people thinking that may smooth over what seems like an immediate future full of legal challenges, particularly in Europe and some other people thinking that as long as all the newcomers to the Twitter corpse party are interoperable they can flex their superior resources and development.
Because that's how groups of people behave.
But I'm also very confident that nobody looked at the rounding error that is the fediverse userbase, disproportionally made up of FOSS true believers and fringe infosec nerds and went "we need to plot their demise". That's not a thing that groups of people concerned with building userbases in the billions talk about.