Right. The OP isn’t asking for a term to describe Moore. The OP is asking how to describe the use of this particular term–whether it be correct or not.
There are two separate questions being addressed. The OP’s question is “what do you call the use of a term to apply to someone who predates that term?” Anachronism or prolepsis seem appropriate.
The second question that is what many are answering is “What term can be applied to someone that predates the era that the term arises?” Sticking “proto-” onto the front is the popular answer.
If the concept of a housewife as an archetype can be an anachronism, then I don’t see how the concept of a Marxist pre-Marx would not also be an anachronism. It’s an anachronistic use of the term Marxist.
The person is committing prolepsis. Is that an error, or just a description of the act? Sort of like an oxymoron - something that seems to contradict itself. In this case, something that seems to predate itself, e.g. “precolonial United States”.
I do agree one should explain the error. “It would be difficult for Thomas More to be a follower of Carl Marx, given he lived a couple hundred years before Marx.”
Of course, language isn’t linear. The concept of communism as described by Marx is called Marxism, but the concept might have predated Marx, or at least precursors. He’s just recognized for formalizing it. Would calling More’s Utopia as “communist” be out of place*, or would that also be an out-of-time usage?
I’m not suggesting More would have used the word, but that we apply the word in a backwards way to say the ideas are such and such, precursors to the formalism of Marx.