Georges Sorel was a Marxist? Yes, albeit certainly not a mainstream one. His theories about "myth" do not sound Marxist at all; they are at most a deviation from Marxism. His idealisation of violence may be rooted in Marx's famous quotes about violence being a necessary part of historic change, but it is also something different: for Marx, and most Marxists, violence is a tool, albeit an inavoidable one; for Sorel (as for Mussolini) violence seems to be something good in itself.
His biography includes cooperation with Maurras' Action Française; how many other Marxists have cooperated with Maurras, or even failed to decry Maurras' actions and aims?
On the other hand, Sorel vehemently opposed WWI, while Mussolini broke with the Italian Socialist Party exactly because he wanted Italy to take part in the war.
So neither the connection of Sorel to Marxism is straightforward, nor is his connection to Mussolini as solid as the latter would have us believe (Mussolini, by the way, was quite the liar; did he really owe anything to Sorel, or did he just found it useful to attribute some of his ideas to someone of higher intellectual reputation?)
Lenin, Rosa Luxemburg, Trotsky, Hilferding, Kautsky, Labriola, Gramsci, Talheimer, Bukharin, Bela Kun, Clara Zetkin, etc., were also Marxist - and each of them more clearly associated with the most important trends and controversies between Marxists in the first post-war. Are there any quotes of Mussolini aproving of these people? Has Mussolini ever recognised any debt to Rosa Luxemburg or Labriola?
And the Nazis claimed to be in intellectual debt to Nietzsche. Does this make Nazism descended from Nietzscheanism? Would such association even make Nietzsche happy? Or, as hinted by his controversy with Wagner, he would have repudiated Nazism as just another kind of vulgarity?
There is a difference between filogenetic connection and functional similarity. A bat is a mammal, not a bird, but a bat flies like most birds and unlike most mammals.
These questions about fascism and communism tend to confuse those things: if someone was a Marxist but then turned fascist, then fascism must be rooted in Marxism. If one is rooted in the other, then they must have similar functions and consequences. If both fascists and Marxists are adept of political violence, they are similar; if they are similar, then one must have taken the idea from the other, or be descended from the other.
This line of reasoning is usually reserved for things we dislike. Jim Jones obviously drank from the Bible and from Christian tradition - however we don't expect the Pope or Ted Cruz to lead a collective suicide just because they have those common roots with the People's Temple. We would rather speak of a corruption of Christianism, or Protestantism in Jones' case.
Small d democrats often decry Stalin's (or Ataturk's) forcible relocation of entire ethnicities, but a liberal democracy did something very similar before (the Trails of Tears) without we concluding that Stalinism is ideologically descended from liberal democracy.
In the case in discussion, I would say that there is double corruption: Sorel's Marxism is an already corrupted version of Marxism, and Mussolini's use of Sorel's decontextualised phrases and sentences are even more transparently a corruption of Sorel's thought.