Unfortunately even if you did gain access to the Steam account - a platform accounts link to a Bungie.net profile became permanent on March 25th so you cant unlink it regardless. Bungie won't forcibly unlink the account and they aren't granting any exceptions to it. What it looks like has happened is the account has been linked in one place with the email/password of the steam account and in another place using the unique account name and password. Steam allow multiple accounts under the same email so you aren't necessarily linking the correct account using the email.