Comment by isomorphic

My interpretation:

First, they can't attack a WiFi access point for which they do not know any password(s). Thus your multi-SSID access point with multiple passwords is "safe" from this particular attack.

However, second, they can attack an access point for which they know any password, gaining access to clients on the other SSIDs. This means your security is now effectively only the security of your worst SSID's password. It also may defeat your purpose in having multiple SSIDs/passwords in the first place.