Running does not make someone a good grappler.
Training and practice can make someone a good grappler.
A small woman who is well-trained and practiced can flip much larger men - that is why this is used for self-defence. Testosterone has absolutely nothing to do with it.
My friend's almost five year old daugher has been doing MMA for a year - she's got mad flipping skills with very limited testosterone.
Your boyfriend was an untrained person up against a trained person. It was a great demonstration of how this works as a self-defence tool.