Muten Roshi never even said to thrust out your arms, or even suggested that they should go into horse stance....Goku just shot that sh**
The whole idea of a
Kamehameha form only exists because they draw a similar pose every time a character does it, I don't believe any character in the fiction ever specified that you should even shoot it from your hands
So yeah, since it was never said, I suppose someone could pose Goku firing from his butt and it could be correct. My mistake to insinuate that there was only 1 way to do it, I only meant to say that the arm bent with chest to the side was the
"common" way to do it, I should have checked my wording :/