The Voynich Manuscript is a cipher because...?

[david]

I'll agree that a complex cypher is improbable around the time the VMS was made. The ones I've seen from that time period are pretty straightforward.

But what Mozart did was improbable too—composing symphonies at the age of four. It's possible he was terrible at art, terrible at business, terrible at business, terrible at many things, but he had a gift for music. Maybe the VMS author had a gift for certain kinds of symbolic thinking.

That simile doesn't work. Mozart was just exploring something that already existed, he didn't "create" new musical notes.
I can accept that the VM is a cunning cipher. But I doubt very much that the Scribe managed to leapfrog several hundred years of linguistic, semiotic and mathematical development to create a new type of unbreakable cipher. He just didn't have the tools at his disposal to make that sort of breakthrough.