For the record, the numbers they reported were based on the number of Instagram users that downloaded the Threads app and took the active step of activating their account there. Threads and Instagram share accounts, so it's a very seamless process. What Meta very much did not do was take the number of Instagram accounts, which is around 1 billion, and say "We have a billion Threads signups!".
Sharing accounts and thus making it extremely easy to sign up for was, if anything, a very clever and smart move.