Most guys have no idea what they're talking about when it comes to bodybuilding "genetics". Bodybuilding genetics has nothing to do with how big or muscular you are as a kid. Everyone looks he same as children, with the exception of a select few. That doesn't make those few anymore capable as bodybuilders, they just have a better base to work from.
Bodybuilding genetics is about muscle insertions, predominance of type I or type II muscle fibers, the size of the heart and its ability to pump blood, and an individuals ability to metabolize large amounts of AAS without severe side effects. These videos that show pictures of guys as children and then pics as swole-as-fuck adults as "proof" of anything are blowing smoke up your ass.
At the absolute top level of competitive bodybuilding, genetics definitely play a role. Is it the line of demarcation between winners and losers? Certainly not. In fact, the overall role of genetics as it pertains to bodybuilding, or sports in general, is far too complicated to be analyzed and definitely stated, in either direction, in a bullshit 5 minute YouTube video.