Pure bullshit. 4L is for speed control, 4H is when speed is not a factor, 2H when traction is not a factor.
With the dreaded 3.07s in my YJ I ran up the mountain in 4H in second gear and came down in 4L in third gear. Pick the gearing which gives you control.
With my TJR, I will often pick 4L towing a trailer across flat solid rough ground just to keep the trailer in control.
PS, 4WD and lockers let you TREAD LIGHTLY and drive without disturbing the watershed, without needing momentum, without tire spin. 4L and lockers are the holy grail of TREADING LIGHTLY. 4H does the same thing.when you drive quicker. 100HP to 2 rear wheels is 50HP each. 100HP to 4 tires is 25HP each...... which one is more likely to spin a tires, destroy the tera firma, piss off the locals and get the road gated.
THERE IS NO HARD FAST RULE. Drive to avoid wheel spin, use the gearing available to maximize traction.