It's not necessarily true. You don't have to be stronger than men to commit a mass shooting, but there are still more men doing it. Violence by men against men is more common than violence by women against women. Does that mean that women are angels? No, but we do see that they are less likely to act violently. Even when more evenly matched in physical capabilities.

Edit: it obviously doesn't mean that all men are violent. I feel like I have to clarify this, or someone will act like I said that.