I don’t believe they were referring to unique email accounts, but to unique accounts in their password manager.

Which means a few hundred accounts to move to a new email address.

The reason I use Proton is to have a unique email for every account that cannot be used to find my original email address, and that I can delete at a moments notice if said email somehow gets on a spam list. Nobody except me knows my main account email. The + method does nothing for me.