This is a ludicrous argument.
You're free to argue that actual Marxist socialism is somehow impossible, given that it has not been demonstrated other than at a small scale. That's a matter of opinion.
But the failure of these regimes does not change the fact that they were so far ideologically divorced from Marxist socialism and communism that most of them have met armed resistance from socialists and communists.
And does not change the fact that the name has been used by groups with diametrically opposed world views.
The point was not to argue whether or not socialism is possible or viable in any particular form, but to point out why communism gets treated differently from nazism:
There are only totalitarian nazis, while among people who call themselves communists you find people ranging from totalitarian via supporters of parliamentary democracy, to anarchists, and within those groups the former have throughout the last century been among the ones who have murdered most of the latter.
E.g. during the Spanish Civil War, Stalins Comintern agents were more successful at murdering high-ranking anti-Stalinist communist leaders than Franco's fascists was.
Anti-Stalinist socialists and communists are as opposed, or more so, to the type of regimes you talk about as they are to the nazis or facists
I've personally faced Stalinists who believed I belonged in labour camps because my views on socialism includes a firm belief in democracy, and I've personally made it clear to very same people that if they were to ever gain power, I'll take up arms against them same as I would against fascism or nazism.