That's actually bullshit
Plenty of great jabbers mainly use it to begin combinations.... because it's the fucking jab. That's just a giant "no shit". That's also false, he uses it at the end of combinations plenty.
His right hook is absolutely money, and in his most recent run he's absolutely been mauling dudes with it... after establishing his jab
His left hand? Monstrous, and he lands it.... off the jab
Robbie has a great jab for his style, it might not be text book but it's fun and diverse. He uses it to dictate both range and tempo, once it's established it remains effective via feints and good footwork. He'll throw it to the body if he needs to, and he'll use it off of covering the opponents lead hand.
Watch his most recent run - His jab has been pivotal to many of the striking exchanges he's one, whether that means the jab itself of the threat of it. There's a reason why the dynamic of the second Rory fight was opposite from their first, and why he was able to beat Hendricks to the punch whenever he wasn't taking minutes off.
Even in the Condit fight (where it was mysteriously absent), it showed itself in the 5th round and in that round he finally started landing his hook and straight that you praise.
We can debate whether it's his most important weapon, but the fact that it's up there is just that. A fact. Watching his fights themselves proves this.
He's a vicious southpaw puncher with a great jab.