Honestly I dont think it's a sequencing issue, its just that his slider is wildly inconsistent and he has no idea where it's going to end up. I also think he's tipping it though I have no idea how to prove that without a side by side video, which I don't have the knowledge or software to make.
He's getting 0 swings and misses with it, so it's like they know right away if it's going to be a hittable stirke or not. Anything even close to the zone they are laying off it without even a half swing or check swing.