When you open a Threads account, you can use your Instagram credentials to sign up if you want to bring over your profile and your circles from the photo-sharing app.
I think you can deactivate your Threads profile, at which point it wouldn’t really have an impact on you (I’m not sure, I don’t actually use Threads or Instagram). And they’re very clearly branding Threads as “an Instagram app”, so the fact that it uses an Instagram account isn’t too surprising. Having two separate accounts could work too, but at the moment it doesn’t really seem unexpected or detrimental that they’re the same account.
I think you can deactivate your Threads profile, at which point it wouldn’t really have an impact on you (I’m not sure, I don’t actually use Threads or Instagram). And they’re very clearly branding Threads as “an Instagram app”, so the fact that it uses an Instagram account isn’t too surprising. Having two separate accounts could work too, but at the moment it doesn’t really seem unexpected or detrimental that they’re the same account.