It's luck/not enough time on the insulin/gh coctail (still young). Luck because of the structure with the typical-for-short-guys long waist/short leg look. Easier to keep the waist down.But look at the back double bicep. It's not 80s quality skin. He's on all the same shit as the rest. Not saying you don't know this perfectly well, but making the point that Flex doesn't look the way he does because of old-school methods.
Thanks for the updates Meso_z .