Kyle, good Q! We find this cue to be challenging for the athlete.
First, in this position you must find stability from the core / pelvis. When you squeeze your hands together you are finding stability in places we aren’t necessarily trying to train with the movement.
Second, it cues external rotation at the shoulder which slightly opens the region around the diaphragm, allowing for a better and more open position.
Last, it resembles railroad tracks at the arms. We want to train ourselves to be railroad tracks at arms and legs as often as possible. Crossing the arms in sprinting is no good. Why not train the ability to stay open and in upper body railroads while planking!
Have you tried it? Did you notice any difference in difficulty compared to how you are used to doing it?