There are tons of ActivityPub implementations out there already
but none are widely used by such a massive amount of people as threads, and especially people who don't understand/care about spec compliance or even how federation works
honestly, i think in the best scenario, threads will create their own activitypub "fork", and most instances won't want to follow it, forcing the people who were on non-threads instances to chose between going to threads to keep in touch with their threads mutuals, or staying on non-threads instances and no longer having a reliable way of keeping in touch with those people.
worst case would be instances following what meta does and making them the spec dictators pretty much, the spec would become closed source and all other fedi implementations would lag behind in features compared to threads, and they can at any point change the spec and break other instances.
i think the point of defederating with threads isn't just the defederation, but is about sending a message that we don't want to play their game, we want to keep doing our things our ways. if they want to interract with the fediverse, they'll have to play by our rules, we don't want to follow theirs