Threads isn’t currently using ActivityPub, but vocally expressed that they are planning to in a future release, in order to “join the fediverse”. They have not expressed when, or what people will and won’t be able to do, or what the business goal is.
About the latter, some are speculating that this is a typical attempt of a closed-source editor to pretend to join an open-source effort in order to destroy it, as has happened in the past. It could realistically be that, or something else, no one knows; but that explains why people are calling to defederate Threads when that becomes real.