It seems to me the author does answer the question in this passage.
"The decision of where to stop does not simply come down to what may or may not disturb some buttoned-up sensibilities but, given the intentions of the fighters and how they go about achieving their aim, where it should stop is when it becomes obviously, dangerously violent."
Of course "obviously, dangerously violent" is rather subjective and a poor description from someone who doesn't understand the sport. Clearly the author thought nine unanswered punches was excessive, but where is the line? Eight, seven six?
As I understand it most MMA rules allow the ref to stop the match if, at any time, a fighter is not defending themselves. From the description in the article, I would tend to conclude that the ref failed in that regard, but I would have to see the actual fight to make an informed judgement.
Getting back to "obviously, dangerously violent", let's examine that statement. First off, the word "violent". The whole match is violent so there's a problem right there, so if we have to couple with "dangerously" we are looking for "dangerously violent" but again that's subjective and in many people's opinion would describe the whole fight. Lastly I think the word "obviously" presents a problem. Obvious to whom? The ref? The fight doctor? A journalist with no combative sports education? I would argue that even if the danger is NOT obvious the fight might have to stop regardless, based on the ref's informed opinion of the situation.
The ref must have the freedom and authority to stop the match at any time. Promotions must be very careful about putting any pressure on refs to let fights run or avoid "controversial" stoppages. Let's face it. No stoppage can ever be as controversial as a death or permanent injury. If the fans the or the media or the fighter's camp ever question a stoppage the promotion has to stand firmly behind the ref and defend their actions. Refs have to KNOW that the promotion has their back or they are going to second guess themselves in the ring. The fighters and trainers and journalists have to know and accept this as a fact of life and just shrug if they ever think a stoppage was too quick.
As I see it, the ref's priorities are (in order)
1 Saving the loser's life.
2 Ensuring a fair outcome.
3 Enabling an entertaining match.
You should not put 2 before 3 and you must NEVER put 2 or 3 before 1.