Washington was respected by his enemies, who knew him and his tactics quite well. A great deal of the respect for Mandela is accompanied by ignorance of the man and his tactics.
Mandela does deserve respect as a symbol of resistance to racial injustice. He also deserves a good deal more criticism than he receives.
I more or less agree on Mandela, actually. No one is above criticism.
As for the respect that the enemy had for George Washington, you're buying into a bit of an ahistorical reading of the situation. The truth is, if you research the 18th century, and especially the Brits, you'll find that there's a pretty foreign (to us, anyway) class dynamic going on.
If you captured a general or even a lieutenant (both of whom would have been born gentlemen) you would bring him home and wine him and dine him and show him how civilized you were because, of course, it would be rude to do anything else. He'd roam the grounds with your family and you wouldn't even lock his bedroom door. In return, he wouldn't try to escape, because that would also be rude.
None of which meant that you wouldn't hang him once you had convicted him of treason, or that you would hesitate to run a lower class soldier through with a bayonet.
The whole 'respect' thing was nothing more than a way for the upper classes to demonstrate that they had the taste and sensitivity to recognize 'good breading' when they saw it (which, when you think of it, is a pretty necessary part of the game if you want people to buy into the idea that 'good breading' is actually a thing).
The idea that there was any sort of circumstance under which the racist white power structure of south African apartheid would ever express the same kinds of sentiments about a black resistance fighter is a little rich.