In this case the added layer is one of wet tissue paper, at best. Defense-in-depth is only effective when the different layers are actually somewhat secure in their own right.
It's like trying to argue that running encrypted data through ROT13 is worthwhile because "well it's another layer, right?".
> you'd be thanking all available Gods that you at least made the IDs not guassable - which would also give them also access to every user account on the system.
I wouldn't be thanking any gods, because no matter what those IDs look like, the only responsible thing in such a situation is to assume that an attacker does have access to every user account on the system. Moving from sequential IDs to something "hard" like UUIDs only delays the inevitable - and the extraordinarily narrow window in which that delay is actually relevant ain't worth considering in the grand scheme of things. Moving from sequential IDs to something like usernames ain't even really an improvement at all, but more of a tradeoff; yeah, you make life slightly harder for someone trying to target all users, but you also make life much easier for someone trying to target a specific user (since now the attacker can guess the username directly - say, based on other known accounts - instead of having to iterate through opaque IDs in the hopes of exposing said username).