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?
£5 for an 8 pack of coke zero decaf at the supermarket, I just checked. What and where are you looking at?