I think you underestimate defensive backs in this league. Contrary to popular belief, you do need some sort of separation to make a catch. Unless you are Odell Beckham.
I'm not underestimating them, but Broyles doesn't need to be an all world sprinter to get open. Good route runners like him can find ways to get open.
Maybe his knees won't ever hold up through his hard cuts? I dunno, but my point remains.