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.
Presumably they would have created ~2 Billion Threads accounts since there are ~2B Instagram users. Even if it was just the US there are approximately ~115M Instagram account.
So no, the 70M user number would just be the number to actually try Threads.