Lassiter open (using Epps as a decoy). That's how it looked to me.
I do think you're right that if the KSU defenders had played it like the Wyoming defenders did, then Epps would have run outside and Retzlaff would have thrown him the ball for an easy score outside. My guess is that BYU was expecting KSU to learn from Wyoming's mistake, leaving the slant to Lassiter open (meaning the throw to Epps would have been the backup plan).
But us fans cannot really know who was the first option and who was the second option unless one of the players or coaches tells us. So that is just a guess on my part.
Either way, it was a great play.