The fracturing and friction of coffee beans during grinding generates electricity that causes coffee particles to clump together and stick to the grinder. Researchers report Dec. 6 in the journal Matter that coffee beans with higher internal moisture produce less static electricity, which means less coffee is wasted and there is less mess to clean up.
Very tiny amounts of salt will also make low quality bitter coffee a lot more palatable and help you use less sugar, so I assume the sourness of the lemon probably has a similar effect. And when I say low amounts I mean like maybe 50mg of it for a large cup.
Oh who did a video on that? I forget. Anyway, what the experimentation showed was that it was only really useful in cutting the bitterness in really bad coffee (as you described), and that the amount of salt to add to a single cup was something like three or four grains of table salt.