I have a few thoughts on this "theory".
If the point of charging to only 80% is to get a few more cycles, OK lets say for a moment that it does work, and you get 20% more cycles with the same amount of capacity remaining after say 800/1000 cycles, I say what's the point ? You no doubt got less total miles out of the battery only charged to 80%, because the "top" part of the battery is by far the most powerful, and the bottom part of the battery the weakest, and quite prone to voltage sag under load.
The manufactures all seem to agree that charging individual cells to 4.2 is perfectly fine. What "some guys" think, and gets repeated over, and over the internet don't interest me too much
I also wonder if these smart chargers also balance the cells at a lower voltage or just shut off at some predetermined voltage. Myself I'd prefer to have my cells balanced at least every 2nd or 3rd charge. I know my "cheap" charger takes about an hour to do this, the battery is up to full voltage long before the light turns green.
On top of that most of my rides have averaged 30 miles, and I want/need all my battery.
It is a good idea not to charge to full, and leave in that state. I like to top it up as close to ride time as possible.
Luna cycle sells a charger for 48 V systems that will charge to 80, 90, or 100% and is reasonably priced. The cycle satiator, which I don't think is quite ready for sale just yet. It is priced at about the same as a new battery ? Hardly worth it, IMO except for the fact it is adjustable, and could charge batteries of different voltages.
I guess having said all that if you know your only going for a shorter ride there is no need to "fill it up"