I think a good prosecutor (if the MMA guys are the defendants) would use the law enforcement analogy and ask, "did you not think that they no longer posed a threat? Wouldn't a reasonable person then de-escalate at that point?"They'll use that against them saying they used excessive force and should have left when they could have. Just like they would do if a police officer did it.
Self defense has its limits. It could go either way depending on what transpired up to that point.