RNG done right (Fight Night Champion) is still absolutely skill based. Having a 50% chance to KO someone, even under very specific perimeters, is still RNG.
You are absolutely correct when you say nothing in MMA is random in reality. The issue is, no one has found a way to replicate feeling of "anything can happen" except with RNG. Which I still think is a very capable system when done right. This would include skill checks like you mention, counters, perfect distance shots, ect.
Kind of like Fight Night Champion (I keep mentioning Fight Night because its striking is largely celebrated by this community despite the fact it used the big no-no........RNG)