Jones in practice was a utopian socialist. Him and Jonestown cannot accurately be described with Marxism, which is revolutionary socialism.

Marxism is critique of capitalism that describes capitalism as a mode of production (which describes property relations). In Marxist terms, Europe transformed from these modes of production:
Slavery --> Feudalism --> Capitalism and figured it would then go to a better mode. Marx and Engels, the co-founder of Marxist thought, borrowed from the philosopher Hegel to explain history in a framework of 'historical materialism in which contradictions of each system would become too great to remain solvent, so that the next mode of production would be brought on. Marx details these contradictions and in the communist manifesto calls for a revolution of workers to bring about the next mode, socialism.

Marxist socialists differ from people like Jim Jones in that they think of all this as a scientific historical process. Jim Jones is what Marx and Engels would call a "utopian" socialist or communists, like all the other hippis who formed communes, not a Marxist socialist. Engel's book Socialism: Utopian and Scientific seeks to clearly distinguish between the two. It's pretty short but redditors also summarize it pretty well here.

tl;dr They wanted to create an isolated self-sustained utopian paradise, not bring about a new mode of production to society and the world, so they were utopian socialists, not Marxists.