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Deleting your Threads account also deletes your Instagram, Meta working quickly on the fix
(www.engadget.com)
This is a most excellent place for technology news and articles.
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.