On second watching, I could see Danza's head snap up when Mir's knee first strikes him, even though on the video of the follow through, the knee more clearly appears to connect to the chest. In realtime the action happens so quick that it is hard to see.
However, I'm somewhat surprised that it was not simply a warning (the first time Tyson bit Holyfield's ear he was only warned), but I don't know all the rules for that style of fighting.
For example, on a low blow in boxing, a fighter can continue if he wants to after being allowed some time to recover.
At least Mir did not appear to strike with a knee for any reason other than instinct, since he already appeared to have Danza reeling from his punches.
ADG