I'm not sure order-3 Markov chain is "simple". Can you explain how would the XV century scribes implement it? For starters, this requires some well designed state transitions, and a lot (thousands?) of them. It's one thing to approximate some sequence of characters using a Markov chain, it's a different thing to explain how this sequence of characters was produced.
Also, your generated sample, which you describe as "indistinguishable from Voynichese to the eye" appears a bit odd to me:
qoeeal shd shos lsheodain oteedy dchdar ol ar okold qokal qopchdy or olkchy qokeey yteey otair deeol shar chedy chor ykodaiin
It's reasonably good, which is of little surprise given the order-3 Markov chain, however I don't think qoeeal looks very plausible (it doesn't appear in the MS), and neither does lsheodain or ykodaiin, though they look less iffy, while okold doesn't appear as a standalone word, dchdar is quite rare, deeol only appears once as a standalone word and once as a prefix. So, to my eye this looks like very unlikely Voynichese, because it's packed with rare and unusual words.
Also, your generated sample, which you describe as "indistinguishable from Voynichese to the eye" appears a bit odd to me:
qoeeal shd shos lsheodain oteedy dchdar ol ar okold qokal qopchdy or olkchy qokeey yteey otair deeol shar chedy chor ykodaiin
It's reasonably good, which is of little surprise given the order-3 Markov chain, however I don't think qoeeal looks very plausible (it doesn't appear in the MS), and neither does lsheodain or ykodaiin, though they look less iffy, while okold doesn't appear as a standalone word, dchdar is quite rare, deeol only appears once as a standalone word and once as a prefix. So, to my eye this looks like very unlikely Voynichese, because it's packed with rare and unusual words.
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