It wasn't automatic. Not sure where you got that idea. But you had to get the app and then sign in using your Instagram account to set up the thread profile. They had "shadow copies" of your Instagram profiles on threads so for example you could sub to someone's profile and when they'd join threads you'd get their activities. But no it wasn't automatic.

Upvoted for your edit. Publicly admitting you're wrong has become too rare.

This is the key. There certainly were NOT 100mm people that signed up of their own accord.

To add further context to the corrections this comment has already received, there are 1.6 billion (with a B) Instagram users. Far more than 100 million.

It was super easy to sign up but ultimately it’s missing a lot of features and is why people didn’t stick around. There’s still no search or hashtags so finding content that interests you is basically luck with the algorithm.

While it wasn’t entirely automatic they’ve showed ads for it in Instagram that easily fooled you into assuming, that the only way you can continue using Instagram was by creating a Threads Account.