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. The whole reason why Facebook is making Threads ActivityPub is so they don't get hit by EU rules about being gatekeepers of content.
This means your normal fediverse apps (e.g. Fedilab) would be able to work with Threads natively, without any need for "read-only" instances like you say.