I don't think they'll force a specific API, but the DMA forces messaging companies to interoperate and I wouldn't think Twitter would be exempted.
I wouldn't be surprised if the EU would force Twitter to interoperate with ActivityPub if they go so far as to disable API access. The EU already has their own Mastodon server so some people already have knowledge of the possibilities of decentralised social media and they also have a huge anti-big-tech stance.
> Can't we have the EU force them to provide an API and thus interoperability with competitors?
Maybe, but it took them over 4 years to rule that Facebook was breaching the GDPR even though a just a casual reading of the legislation would've given you this answer.
Maybe they'll "force" them... in so long that Twitter may not even exist anymore.
Also, the whole silo thing reminds me of the old telephony days. We got over that, so what is the problem now?