The Constructed Language Hypothesis

[Hermes777]

The starting point for Leibniz was Ramon Llull's Ars Combinatoria. He saw the seeds of an artificial language in Llull, and this provided impetus for later growing interest in artificial languages. This would be a good starting point for an adventurous mind in the early 1400s too. There is certainly an abundance of evidence in Voynichese suggesting combinatorics. (The other possible source would be the Prophetic Kabbalah of Abulafia (also from Spain) - a system of combinatorics and an incipient 'language' (vocabulary) from the 72 Letter theonym.) 

Modern conlangs, Esperanto, Tolkein's tongues etc, are properly linguistic, credible imitations of natural languages. Voynichese seems far more mechanical than organic. If the question is how our author might have approached creating a constructed language, Llull's systems and similar suggest themselves in the context: there is a Llullian revival in the first half of the 1400s.