The shoe doesn't fit, though. That's not the meaning of the term.

A true Marxist doesn't support authoritarian governments

Stalinism operated under the auspices of Marxism, as did Maoism. Like it or not, these are Marxists. The problem is they're authoritarian Marxists, not fascists.

And fascists don't call everyone they disagree with "liberal." I myself reject liberalism, which upholds capitalism and other inequitable hierarchies. Shall we call liberals "fascist" as well, since they too engage in oppression and exploitation of marginalized groups? Certainly not, and nor should we call authoritarian leftists "fascists."

Fascism is a strictly right-wing ideology. Far right, in fact, which is why we typically don't call liberals by the term, even though it too is a less extreme right-wing ideology.