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.
There might be things Meta isn't allowed to do with WhatsApp. Also the concept of account is bit blurry on WhatsApp because you basically login with your phone number and an SMS code sent to your device. This wouldn't work as well for a service that can be used elsewhere.