Implement ActivityPub on threads.net. Despite carrying Facebook’s reputation, get federated by relying on a couple of groups: fediverse enthusiasts greedy for growth, and those who are indifferent because they think it will be possible to back out later if it goes wrong.
Get people who are popular on Threads well established with followers on the outside network. This one-way action will flood the fediverse with lots of people and content that were previously unavailable. Anyone who was opposed will look kind of silly in the face of this bounty. Any future attempts by outsiders to defederate will be met with internal pushback “but I’ll lose access to X, Y and Z!” The infighting would sap volunteer energy. Inconsistent federation rules would lead to server-hopping and a drain of users to pro-Threads servers, or Threads itself.
Begin complaining of major problems with spam, trolls, sockpuppets and sybils. The fact that ActivityPub can originate from any domain with any number of users means that you have to rely on reputation and servers’ own moderation policies, which are highly variable. This is entirely incompatible with Meta’s commercial mode of operation where it has to deliver a certain kind of service so as not to spook the customers (that is, the advertisers). Even though these problems are probably occurring, it is easy to fabricate traffic if Threads isn’t suffering the levels of spam required to garner sympathy.
Require that each individual user who wants their content to federate with Threads accepts a terms of service by sending a message to a specific account. The code will be obtained by visiting a Meta website where you enter your email and agree to the acceptable use policy, understanding that you may be blocked later if you fail to comply. For bonus points, make this as low-friction as possible, not even requiring a Facebook account.
Block federation with all users who have not accepted the terms of service. The majority, who want their fediverse experience to continue as it did before, will take the individually-convenient path of signing up with the terms. Users who are blocked will have the frustration of being shadowbanned in conversations when viewed within Threads, adding to the pressure to agree. This will lead to a widespread perception that the non-Threads experience is “broken”. Some servers will defederate despite the warnings of step 2. The fediverse will be irrevocably split between a vassal of Meta, who can claim to regulators that they are interoperable and therefore not anticompetitive, and a handful of nerds who will slouch off to build anew without the others.