Fascism is explicitly militaristic and controlling. You can't have a liberal fascism even in theory. To even desire a fascist world is sufficient evidence that you're a bad person. When fascism kills, there's no remotely plausible way to say "that wasn't real fascism," because that violence is at the very core of fascism.
That is not true of Marxism, and that's the key to the difference. Marxists have never governed without horrific violence, true. But Marx never laid out a plan of violence. He never really laid out a plan at all. There's no roadmap in Marx, just a starting point and an ending point with a couple of suggestions for what you'll see on the journey.
The vaguely theorized endpoint of Marxist theory is peaceful cooperation and the disappearance of government. So the brutality of actual Marxist governments - contra fascism - does not match up to theory.
This means the devout believer in the end goal can always pretend to themselves that there's no problem in their goal, just in the way others have gone about it. "That wasn't real Marxism/communism" may be a self-serving defence, but it's not inherently theoretically wrong.
Of course they make several critical errors. Concerning the journey, they don't understand Hayek's argument about the ruthless type of person it takes to accomplish their goals. Even if Hayek were wrong, the lack of a clear roadmap makes their movement particularly susceptible to co-optation by people who are looking for their own purposes. The journey being a blank space on the map, it really should be labelled, "Here be monsters."
Most fundamentally, they fail to recognize that the destination may need continued enforcement at odds with their theory. Because their view of human nature is that it's entirely socially constructed, they think that a generation or two in a communist society will be sufficient to wipe out any capitalist/marketist tendencies. Unless they are 100% right about that, capitalistic /market behaviour will always be a threat: contra Marx, communism may not be a stable equilibrium.
But the theory is that it is a peaceful equilibrium, so as real-world Marxism has never lived up to this ideal, there is a truth to the claim that "that wasn't true Marxism."
The harder ones to explain are the folks who believe places like East Germany, Cuba, and Venezuela were/are closing in on that Marxist ideal. The evidence of people voting with their feet doesn't phase them. I guess the emigrees are just folks still suffering from false consciousness.
As to Marx's racism, that's just pure double standard. In his case, it's just an unfortunate artefact of another time. But if Smith had used the N-word, in an even earlier time, it would be evidence of his inherent wickedness. But we're all probably somewhat inconsistent in pit judgement of those we like and dislike.