Not to say that Coach Sommer is wrong, since I personally don't think steady state cardio is a killer and I really respect some of the articles he's written. However, I do think that particular example is flawed. The rings are a heavily upper body intensive exercise. Whereas running really isn't. Strength loss from running wouldn't manifest in rings or even most gymnastics events. Larger legs would probably be considered a detriment as they would make leverage more difficult. I know doing front and back levers for me have become a lot more difficult as I've added lower body mass.
A better question and more concrete example would be how much steady state running are the Chinese olympic lifters doing.
Anyway, my perspective on steady state cardio and combat athletes is that pretty much all great fighters put in road work. I'm going to assume if it was hurting their performance, someone would've figured it out by now.