Got to ask, did the cyclist have a helmet on?
I don't know.
But an handy tip from your friendly EMS cycle trainer...
If facing a confrontation and you can't expedite a swift exit.
1. Unclip your lid strap to prevent getting a broken neck of they grab your lid.
2. Dismount. You're at significant positional disadvantage when astride a bike.
3. Keep the bike between you and them, one hand on the bars, other on the seat, leading foot on the frame where the downtube and seat tube meet. If possible with drivetrain facing away from you as you don't want to get yourself entangled in it if things get spicy.
4. If they try to grab the bikes let them have it. Help it on its way by pushing it away with your foot - nothing makes someone's eyes water more than a frame across the shins.
There's all sorts of stuff, techniques for using the bike as a weapon, ground pins using the bike, etc, but it really needs to be demonstrated and practiced so stick to the basics above.
And remember, unless you're really able to take care of yourself, discretion is the better part of valour. The victim here in no way deserved to get hurt - nevertheless, his participation in the situation was willing, and had taken the cowards way out and hopped it he wouldn't have got his head mashed. No point being morally righteous if it means spending the rest of your life in a wheelchair and being spoon fed by a minimum wage carer...or worse.