Incorrect. MMR is more or less translated to your SR because MMR appears to be your standard devation from the mean of all players (thus the -3 and slightly below that for the lowest ranked players to +3 and slightly higher than that for the highest ranked players). MMR is NOT a differentiator between “good” and “bad” players at the same SR and the game doesn’t even have one. It IS possible to get 6 smurfs that are all solo-queueing at SR 2100 vs 6 actual SR 2100 players because the system treats all players that are showing the same SR all the same. It does not care how you got to whatever your rank is.
Anyways, to answer the question of why comp feels like a “dice roll”, it’s because you are human who has 5 human teammates taking on a team of 6 human opponents. Human volatility is a real thing, for better and for worse.