So we have to look at what "attacking a specific ID" even means.
With separate room numbers and PINs, it means you know the room number but not the PIN. Simple enough.
But in the long-id scenario, that means you have part of the ID, but not all of the ID. That's pretty unlikely to happen. Instead, situations where someone would have leaked the room number will take one of two routes: either the person leaks the longer ID, and there is no attacking necessary, or the person realizes that the secret code needs to be secret, and nothing is leaked at all. Either way, attacks on a specific conference ID no longer happen.
With separate room numbers and PINs, it means you know the room number but not the PIN. Simple enough.
But in the long-id scenario, that means you have part of the ID, but not all of the ID. That's pretty unlikely to happen. Instead, situations where someone would have leaked the room number will take one of two routes: either the person leaks the longer ID, and there is no attacking necessary, or the person realizes that the secret code needs to be secret, and nothing is leaked at all. Either way, attacks on a specific conference ID no longer happen.