"Guerrero landed an overhand left, and followed with a series of straight left hands while illegally holding Berto's head with his right hand to score the knockdown"
Guerrero was not illegally holding Berto's head. You can clearly see in the replay that Berto has hooked Guerrero's arm and pinned it to his own head. The referee has a clear view of what is happening and correctly makes no ruling on it as it is perfectly legal to hit while being held (as opposed to holding and hitting)