Automatically creating a shadow account for everyone on Instagram?
Even allowing people to follow that account?
Sounds like they really wanted to push Threads out the door in a big way.
This feature can be achieved very easily by them without making accounts. They already have a list of Instagram usenames. When a new user makes an account on Threads, check the username against a simple list of Instagram usernames and if it matches, throw an error stating the username is not available.
Why does that person need to have an actual account on Threads?