The point was to illustrate that separate drop-down-accounts are handled in a distinct fashion by the game client, and in account management.
If Classic servers are just servers under your account, but not a separate account, sure. If they’re separate accounts then it would require them to actually link those accounts to make the character limit spread out like that.
It’s interesting because it’s made to be free with a regular subscription to modern. We don’t have another WoW game or version to compare to. Trial accounts can make 50 characters (but can’t level past 20), and cost nothing, though. That’s not even temporary storage, as I’ve had starter accounts on my account for years before.
Ultimately the real reason we’re using drop down menus and accounts as an argument is because, unless classic is just a server type and not a separate account, it’s going to be more distinct than just server type. It’s going to have distinct and severe limitations/alterations to the point that it’s not interchangeable with the other accounts.