The claim is that the undercover cop, for no reason, attacked the cyclist from behind. The video shows him doing that, but we don't know why. Maybe just before that the cyclist did something illegal, or maybe the cyclists are right and the undercover was just trying to stir up trouble.
If critical mass cyclists do what they are supposed to do, which is ride around on the street, it is not illegal. However some police appear to hate it, and look for any excuse they can find to arrest the cyclists--over traffic violations usually.
It does seem to me that using that much force in response to a traffic violation is excessive but it's also true the video doesn't show what led up to it. It's possible that moments before that the cyclist assaulted an officer or some such--it's unclear from the video.