I believe Instagram usernames are reserved on Threads to ensure people can keep the same handle on both networks, but the users aren’t automatically signed up with a profile.
even if that were true - it wouldn't make sense. That would would a) create a huge disparity between signups and DAU b) they did make the signup process very frictionless which would explain the signups better and c) I believe it threads signups were in the millions whereas Instagram has over 1B accounts