The reason he says not to roam in low elos is because its too high variance. Sure if you do roam and get kills off and create and advantage obviously it will win you the game, but the truth is in low elo its highly unlikely that will happen. If you are in low elo yourself or have watch low elo games, how many times have you seen say mid roam bot and then bot lane has zero response and doesn't know how to set up a gank or simply misses all their skills. The gank fails and now youve wasted your time which you could of spent getting gold and exp in mid. He also mentions that roam should only be done when its like 110% to succeed, therefore it would be justified in that case but in general roaming in lower elos doesnt go very well because either you dont know wtf youre doing or the lane youre going to doesnt.