Well if they are only saying that while you step jab do not chamber the rear hand, then they are being completely honest. Because, the opponent may counter or KO you—as Teddy Atlas teaches
in this video. Also, it's true that it takes longer to punch the opponent's face when chambering the rear hand.
Chambering the rear hand
after it's thrown is not as risky, since the jab is used to put the opponent out of position (double weight). However, it is more defensively responsible to protect your position by using the rear hand to help control the centerline, not chamber it, in case you miss.
However
at 2:29, the Machidas demonstrate to jab
while not chambering the rear hand— which is what Lyoto Machida does in your clip.
In your clip, Machida jabs and does not chamber the rear hand, lands the rear hand on Jon Jones' face and enters the clinch by grabbing Jones' arm with his lead hand.