I don’t think anyone is questioning your cynicism of Meta’s intentions or motivations, but the nature of the Fediverse is specifically designed to make it very difficult (if not impossible) for any one party to control the entire thing.

The worst thing I could see is something like the development of React where FB has an overwhelming advantage in sheer resources and ends up having a major influence on the direction of software trends. But that would still just be a popularity thing and would not actively stop anyone from doing their own thing. Maybe there is something in the license for ActivityPub that would let them pull a Google-vs-Oracle reverse engineering, but again that won’t stop other instances or developers from ignoring them if they wanted.