It's not uncommon for players to come in saying they worked on a pitch in the off season only for that pitch not to make it into games. Devin had a slider/cutter the entire way through the system but I think he kinda set it aside as he tried to get to the big leagues with his other 2 offerings. So I'll hope for the best but generally going to take this with a grain of salt.
Adding it would certainly help, because as good as good changeup is, you generally can't be successful long term against both handed hitters with just 2 offerings. If this pitch is more of a hard cutter, he probably can effectively use all 3 pitchers against all hitters. If it's more of a slow slider it will probably be mostly limited to rh hitters.