Here's another article about 120 rpm cadence.
https://www.roadbikerider.com/whats-...cling-cadence/
I myself doubt they really mean "efficiency" as-is. I think it's more of a combination of actual efficiency and muscle fatigue. If you're quite well trained at high cadence, you'll be able to cope with muscle fatigue better. At least that was my experience. When I attempted to do a century with previous training plan of 7 hours per week at preferred cadence averaging 70 rpm, I bonked hard. I completed the trip but felt really bad.
I maintained my 7 hrs weekly training but progressively raised the cadence while retaining the same heart rate intervals to see if things will improve and it did for half-century rides. Training for higher cadence took more than 2 months. I was averaging 110 rpm in the flats by the time I did my first century without stopping and without bonking. I did my hurt my neck because I wasn't used to riding long hours without stopping but my legs are still in good shape at the end of the trip and not fatigued. Training for high cadence does help to a great degree with endurance and delaying muscle fatigue especially if you have very limited time for training.