I'd make a strong division between the two cases you thought of, which might be useful to answering your question.

Marxism is pretty darned complicated. It relies on a very particular set of assumptions and is built on top of Hegelian phenomenology, frequently using Hegelian dialectic to do so. It's often boiled down to a few sentences, but in every case this is reductionist. Even if you just start with assumptions, the particular set of assumptions is large and original to Hegel. It's very unlikely someone before Marx thought the exact same set of ideas. Marxism is not its conclusion because its conclusion is trivial in most schools of political thought - no one would compete for resources in a world without scarce resources.

Rationalism effectively requires two assumptions/axioms/whatever - a definition of 'reason,' and belief in the supremacy of reason. Now, this question is hard to answer for other reasons - rationality is an old concept. You could talk about the first modern rationalist by identifying what the modern definition of 'reason' is. You could talk about the first rationalist ever if you have compelling evidence that there is a core definition of 'reason' which can be traced to a single individual. In that case, your argument is as accurate as your definition.

Really, most things like this boil down to how you define them and what assumptions you run under. These statements contain certain information about what writer Z thinks about X and Y, and Marxism and rationalism. You can get into some grey area - whereas Marxism is strongly defined by Marx's work, rationalism is a bit more open to interpretation, just by the word's construction in the English language.

One actual use for these statements is to denote lineages. To say "Possessed was the first death metal band" is to imply that, if you follow a chain of influences, you will eventually reach a band influenced by Possessed - that nobody independently generated this idea, that their use of this idea is always in reference to someone before them. In the history of philosophy, this is meaningful, because Western philosophy has always been very much in communication with itself.