There's also the regulation angle. The Digital Markets Act is likely why they're federating: https://commission.europa.eu/strategy-and-policy/priorities-2019-2024/europe-fit-digital-age/digital-markets-act-ensuring-fair-and-open-digital-markets_en

Examples of the “do’s” - Gatekeeper platforms will have to:

• allow third parties to inter-operate with the gatekeeper’s own services in certain specific situations
• allow their business users to access the data that they generate in their use of the gatekeeper’s platform
• provide companies advertising on their platform with the tools and information necessary for advertisers and publishers to carry out their own independent verification of their advertisements hosted by the gatekeeper
• allow their business users to promote their offer and conclude contracts with their customers outside the gatekeeper’s platform

The interoperability is the big one. Being federated means that Threads isn't considered a "gatekeeper platform". I wouldn't be surprised if Instagram and maybe even Facebook itself start to federate as well. Since Threads isn't currently connected to the wider fediverse, that's probably why they're not in the EU yet.

This also means that fears of "Embrace, Extend, Extinguish" are likely overblown. Breaking fediverse interoperability means that they'd be a gatekeeper again and subject to EU regulations against gatekeepers. Interestingly, both Twitter and Reddit are now likely subject to being considered gatekeepers due to making their APIs effectively inaccessible.