That is shocking! No, your not being ridiculous, and this isn't what most mothers would do.
If it were my son that had just been hit at the playground, I would tend to him to sooth him and remove him from the situation. I definitely wouldn't take it upon myself to restrain or punish someone else's child. That's appalling behaviour on her part!
Sending you a big cyber *hug* because I'd be pretty rattled by seeing someone man-handle my little boy too.