the null hypothesis for Voynich is that it was generated by some random process, like asemic writing, assembling bits from tables and copy/paste/edit. there are some reasons this is plausible (the entropy of Voynich words is low for language, there aren't scribal corrections etc), but fundamentally the "ancient hoax" hypothesis can't be proven, only disproven (since there's so much freedom of choice in generating garbage that it's not a compact explanation.)

so frustratingly, if it is an ancient hoax we can't ever prove it. all we would observe is that we haven't deciphered it, but failing to decipher isn't proof that there's nothing to decipher.