That idling time is just enough time to let it warm to temp (or close to it) and be sure nothing is malfunctioning. You can’t / shouldn’t fire it up and pull out of the driveway instantly. It HAS to idle for a bit, not necessarily 20 min, I just threw an arbitrary time out there.
Ring tension (which is the ONLY factor involved in ring seating, that can change) is only 5-7 lbs/in inside the cylinder, so at a slow idle there is a considerably slower ring break-in…hence the reason to let it sit and get warm so everything else is where it should be.
After that THEN get on it, load is the only way to raise the ring tension so that they aggressively wear down the microscopic peaks on the cylinder wall in the correct way. You are right that long idling times will smooth out the peaks in the wall, but leave the valleys which will lead to poor rings seal.