Is it true? Let's find out!
The experiment will be carried over on this 2010 Panzer Rally monster truck:
Black edges of the gray strips stick out like a sore thumb. Leaf springs are completely painted by a black marker (alcohol-based Centrofix) and look quite well, but that can't be called edge painting anymore.
Here the marker also didn't work too well. Black edges of light-gray beams stand out and give the model somehow cartoonish look. But the mostly black leaf springs don't look bad.
Tubes made of wooden skewers are painted by water-based black marker. Result: they look like dirty wood, because the paint didn't get in the deepest crevices and got washed away from the very surface. The drive shaft painted by alcohol-based marker looks way better.
The marker excels for the black bumper. The camera highlighted different reflectivity of the two surfaces so the edges are quite apparent, but it looks uniformly black to a naked eye.
Licence plates' edges are black-highlighted intentionally, so they don't count. But the edges of white mudguards, gray armour plates and floodlights over the windscreen don't look good in black.
Another example of unusability of black are the edges of air conditioning unit, side armour plates and the counterweight on the machine gun ring.
This is totally over-the-top: shaky, thick black edges on a completely white rear cabin. We could say that it's been done on purpose to harmonize it with the front cab which is full of black lines too, but in that case the retouching would have to be less prominent.
So there comes a conclusion...
Is edge painting with a black marker comfortable? Yes. Easy? Of course. Sufficient? Only for black parts. Summed and averaged, the final mark is
*** PLAUSIBLE ***
Of course, it depends on every modeler's taste. Non-demanding beginners for whom anything is better than white-shining edges may consider the myth confirmed. Demanding perfectionists probably consider it busted, shattered and stomped into the ground :-).