Yeah, but they’re separate

They’re literally not.

Threads accounts are closely integrated with Instagram accounts. By default, Threads and Instagram accounts share the same username, profile picture, and display name, although the profile picture and display name can be customized. Users can choose which accounts they follow on Instagram will carry over to Threads, either with the other person’s Threads account already created or set to automatically follow them once an account is created.[36] As of August 2023, if a user decides to terminate their Threads account, they must delete their associated Instagram account as well.[37] Adam Mosseri, the CEO of Instagram, has acknowledged this limitation and stated that they are exploring options for a user to delete only their Threads account.[38]

Users are required to have an Instagram account to use Threads. Threads is a companion app to Instagram, and it uses Instagram to authenticate the user’s identity and connect with their network.[32]

https://en.wikipedia.org/wiki/Threads_(social_network)