Yeah I'm sure 5 in a 10 wouldn't fly, I have a half dozen or so 5 rounders I been using.5rds. I'm pretty sure 5rds in a 10rd mag is also no good, think it has to have a block. I have a few MagPul -5 blocks to put in the Gen3 10rd mags.
I'm not sure where these guys got that notion but I can find nothing to say 10 is legal.