He may have felt threatened by his lack of knowledge and realized he couldn't keep up the facade for long. If he could put you at arms length, you wouldn't be able to find out how LITTLE he actually knew about what he is doing with his job.
But to tell someone something so blatant, he must really be desperate. It would be broadcast all over the information network wherever Betta enthusuiasts are if this was the case. Someone in the know would be mentioning it so the newbies wouldn't make a bad mistake. But there is NOTHING. So just disregard this and do what you want. (and perhaps consider finding a different fish store unless you just want to liberate your little friends from this man)
Rose