This is not true. Each Instagram account has the ability to log into Threads. But it’s opt-in. If you don’t want to use Threads then don’t sign up for it. Meta is not duplicating Instagram accounts automatically into Threads.
And yes, you can’t delete your Threads account since it is your Instagram account. Threads is more a feature flag that they turn on/off on your IG account. I agree that they should make it possible to turn that feature off in the future.