Watching on replay from the angle presented, it sure looks like a BS call to me. Max's left hand appears to be loosely held to balance himself for the slide, not a fist or a stiff arm, and it was casual contact. It didn't look like his hand hit or punched the guy, whether in the crotch or not. Whether Max's hand hit the other guy's package is irrelevant vs if he made casual contact on his leg or anywhere else. The fielder's supposed to be wearing protection anyway. Are they saying it wouldn't have been interference if the hand did not make contact with the crotch?
Edit: It could have been called interference on the fielder - interfering with Max's slide. How come it isn't ruled that the fielder interfered with Max?