Why in hell would a massive social media company give two flying shits about a decentralized, comparatively tiny not-quite-one-platform?

To eat it. To completely destroy it and thus gain greater power and control over even more.

Like, the fediverse isn't a threat at all to fb/meta, but people, chatting on internet, without total top down direct oversight on what comes up in your feed?

Well how would they make their money? Or push their propaganda?