Have we ruled out simple substitution unwisely?

[ReneZ]

Yes, one can cherry pick a few similar things for certain languages, but in Voynichese they are systematic (hence the low entropy). Once a certain Jorge Stolfi even wrote a simple grammar that defines the constrained nature of most Voynichese words. I don't know about mandarin, but you cannot do that for European languages.

Also, you had to cherry pick bigrams instead of characters... this tells a lot

All agreed, but in Mandarin Chinese it is also systemic. The constraints may even be stronger than in Voynichese, but that is a subjective impression I have.
From what I know, Vietnamese is quite a bit less constrained, and Thai very significantly less. I know practically nothing about other SE Asian langauges.

(On a side note, Vietnamese and Thai are the only two languages I am aware of where 'ng' is happy to appear at the start of a word.)