At first Skubal only threw a 4 seamer - beginning his 2nd year and then increasing his 3rd, he added a 2 seamer/sinker - so he has two fastballs - if the life on the 4 seamers isn't the greatest, the batter is still going to see FBs going different directions because he can throw both kinds. Melton is about where Skubal was in his 2nd year - he has a sinker but doesn't throw it much yet so that same added confusion might work for him.
It reminds me what a freak Verlander was. He only threw(throws) a 4 seamer but it has so much spin guys couldn't barrel up it even when he didn't throw it hard. And he would vary his FB velo by almost as much as the difference between Skubal's FB and Change, before he even threw an actual secondary pitch.