I found the code that is used with that string, in class X.10G (my jadx renamed it to p00X.C10G). A debug string calls it "FediversePromiseViewModel", and it confirms that the threads.net domain will be the one used for activitypub (or at least webfinger).
#FediBlock threads.net - we have confirmation now!
@Rairii great work! thanks for sharing!