In order to unlink an account, you first have to disable cross save and/or unlink accounts, you must be able to authenticate the accounts. Bungie cannot forcibly de-link them.
I would suggest contacting the platform concerning recovering the platform account.
If you are unable to gain access to your account, then you will not be able to deactivate cross save or unlink it from your current Bungie.net profile.
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I disabled cross save already and authenticated that my Xbox account shouldn't be connected, but when I got to unlink it it says to disable cross save.