EEE wouldn't work on something that is popular. The whole point is to destroy it before it becomes popular. Furthermore, corporations aren't okay with smaller alternatives existing at all. Their goal is to have a monopoly. Finally, Mastodon's growth has been really impressive for the last couple years, so I'm certain that other social media companies are looking for ways to shut them down.

The "gatekeeper" theory has some merit too, but not in that way. You can find the definition of a "gatekeeper" on the European Commission's website and I don't see how federation would affect it at all. That said, gatekeepers are required to "allow end users to install third party apps or app stores that use or interoperate with the operating system of the gatekeeper", and federation would meet that criteria.

Still, we already saw Twitter and Reddit move to paid APIs, and apparently that doesn't violate the DMA, so it's hard to believe that Meta would use a more open protocol without some other motivation.