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.
I think it would then delete any account in both Instagram and Threads.
But there's no need to do that. If you already have Instagram and your happy with it, there's nothing extra you need to worry about just because there's a shadow account pre set up for you in Threads.
It's a shady way for them to push their new service, but it's not like they suddenly have any more info on you than they already had.