Sky Priority - anything but. Just a marketing gimmick.
I was in Atlanta (DL's home for 70 years) a few days ago, sitting in my 1A FC seat departing at 6.30pm - rush hour. We were about number 9 in line for take off. There were tails as far as my eye could see out the window.
It then dawned on me - Sky Priority? Doesn't that logically mean you would have priority getting into the sky? Nope - you need to wait in line just like everyone else. You'd think DL flights would get sky priority, especially in Atlanta.