Yeah he literally didnt teach him jack aside from delivering milk and moving rocks.
I thought that was always flawed, considering Karin even said Goku's movements resemble Roshi's to a T. So I guess all it takes to be a master of martial arts with centuries of fighting experience is to deliver milk and spend a few months rigorously training?
Hell, I'm not even sure Goku is aware he actually fought Roshi when he fought Jackie Chun. At least if you count that Roshi taught him something I guess, since Goku copied Chun's moves in that fight.