Enjolras knows Bahorel well enough that he probably wouldn't have been surprised by that even without the silent pre-agreement and the conversational lead-in. As it is, he's really not.

Bahorel's fast, and cheerfully vicious. But Enjolras is fast too, and joint locks are an excellent way to turn a fight very quickly. With the momentum of that punch, helped along in the right way -- say hi to the mat, Bahorel!