I think the core of this comes down to what defines “new account”. The OP does not have a new account, but has not had paid game time in a LONG time. We know the system is now set to impose restrictions if an account does not have paid game time since 2017 for other things. I strongly suspect, that is the trigger point for “new account” in this case too.
The Battlenet Account may have more than one license on it. If the license in question has not been played since Vanilla it would have no “recent” paid game time on it. Playing other licenses on the Bnet account does not impact restrictions on that individual Vanilla license.