Sometimes there are more variables than right or wrong though, if the person only had an external filter and no spray bar then IMO they would have to get an airstone, you may have only heard part of the conversation.
In the end animates staff are only retail staff, and it would be difficult to know everything about fish if you don't keep them yourselves. Plus fish are not the only thing they have in store.
I'm not saying that they shouldn't be more knowledgeable, but have you been into bunnings, ask anyone in there anything and they can be just as useless.
Some staff make it a priority to learn a bit about the products they sell and some don't. If you are a lowly paid person, which I'll bet most of them are, then you aren't going to give a rats a*se. the only ones with knowledge have been there a bit longer than the others at those chain type stores in my experience. If you find a good one then remember their name for next time.