Not sure who you're referring to.
But in a battle between a southpaw and orthodox fighter, the jabs are usually nullified. It becomes a battle of who can align their power hand with the opponent's target.
Orthodox fighters are used to fighting other orthodox fighters, and the jab is very important in same-stance fights. That's why the Southpaw has a stylistic advantage vs. an orthodox fighter. The orthodox's jabs, which they usually train a lot, is not as effective. The Southpaw's rear hand, which they usually train a lot, is a lot more effective vs. an orthodox fighter.
Watch Manny Pacquiao or Giorgio Petrosyan. These guys had a lot of success despite not being very good jabbers.