Yes, as far as I could say (I have learned this from my t/a, to be honest) A0 (A zero) means you cannot have a waitlist. You would have to have something like AL (A L for waitlist) to be a able to waitlist a passenger.
It depends on which reservation system they used. I have WL on flights that showed A0 before in A Class.
Sometime they have to create the WL booking and then once they end the transaction the system will decide whether you can WL or not (eg. on BA they restict WL in D and I Class)