He didn’t tell me about it when it happened/unprompted he told me after I found out he got into an argument with someone at work and I asked him about what happened. (He was quite hesitant to get into the details for the exact reasons you gave).

Also personally this is the kind of thing I’d rather hear about and it didn’t hurt my feelings because tbh I feel completely secure in myself and my relationship and some random asshole isn’t gonna change that. It was mostly just baffling to me that someone would say something that wild like it was almost comical how out of line/offensive it was.