Do not agree with the bolded part. Do agree with the other part though. Stamina isn't stamina. There are different types. A marathon runner would gas within minutes of being in a MMA cage. I'm not saying doing triathlons doesn't help at all with cardio but if the Diaz brothers conditioning program consisted primarily of trithlons they would gas quite easily and quickly.
This isn't something I've just come up with . I buy Fighters Only magazine every month and there was an article in the training session about conditioning. The dude said that the only thing long distance running is good for is long distance running.
I simplified it to get my point across that's all. Yea, just Cardio is on its own is not ideal, you need the "Muscle stamina" (the sort sprinters have) as well. I dont think triathlons are anything to do with training, they are a barometer for there Cardio-vesicular fitness....
I also think it depends on the style you want to fight, Cardio is preferred for someone like Diaz who grinds you down with light punches. Explosive guys like GSP or Melvin Guillard will be doing more sprints.