Not every black QB is labeled dual-threat. Byron Leftwich, JaMarcus Russell, and Jameis Winston were/are 3 of the most prototypical drop back QBs imaginable. Nobody ever said they couldn't throw the ball.
It isn't a knock on black QBs that they can't throw; it's a knock on dual threat QBs generally that they can't throw. One among many critiques of Johnny Manziel going into the draft was that he couldn't make "NFL throws." Same with Tebow. There were/are some doubts that Mariota can make NFL throws. Dual-threat guys, generally speaking, are stereotyped as not being able to throw but it doesn't have anything to do with race. It just so happens that lots of black QBs are dual-threat.
These types of conversations are part of the Social Justice Warrior path ESPN is taking with their programming now that SportsCenter and a lot of their other shows have been rendered irrelevant.