Sounds to me like they have at the least swung something at your dog. Doesn't have to be a hand smack - could be something like a fly swatter or even the classic rolled up newspaper (or magazine). I'd also pull him from there (and tell them why).
OTOH, the alpha roll is not totally and always bad (would bge in the OP case with a 6mo however!).
A pro trainer and very experienced ScH + K9 trainer had to use it one time with my then about 3yo male GSD who had a bad case of "I am going to kill you" toward another little older male GSD (his half brother!). One roll and some follow up OB and he would at least tolerate the other guy. Trainer said that it was a matter of getting my bully dog's respect and then he would listen to "NO".
So once in a great while, this type of rough correction can work.
BUT, in this case, my guy had a heavy duty muzzle on - and I would never want to try it with an adult GSD w/o such a muzzle - great way to get bitten in the face (and all over!).
My boy was NOT happy!