Now this will get dragged down to what can and cannot be considered the true Left :(
I won't discuss China, the Soviet Union, Che Guevara (where is that quote from, btw?) or anything of the sort here. It simply is not possible or suitable to debate this in a forum like hackernews. Call it a cop out if you want :)
However, I'll say I disagree with your fundamental assertion, that the Left is willing to tolerate open racism when it suits their needs. Racism stands directly against what the Left stands for. Of course corrupted leftist movements, often leftist in name only, are free to forget leftist ideals, just as religious organizations sometimes forget their founding principles.
But let me place emphasis on this: if a leftist movement turns racist, this is a serious contradiction that must be pointed out. If a rightwing movement is racist, there is no contradiction and its intended audience will applaud it; racism and discrimination are cornerstones of rightwing thought. It is a mistake for a leftist movement to tolerate a racist ally when it would temporarily help heir cause, but this is never a mistake for a rightwing movement because racism is often part of its core ideology.
By this definition, which I think is mainstream, it's impossible to consider the Nazis a far left movement. I believe there is an ideological intent to conflate the two extremes, left and right, which says less about them than about the people doing the conflating...