The Voynich manuscript has been pretty reliably dated to the early 15th century by both carbon dating and by comparison of the materials, techniques, and style

The Voynich manuscript has been pretty reliably dated to the early 15th century by both carbon dating and by comparison of the materials, techniques, and style used in the manuscript—tens of thousands of years, at least, after a proto-human language would have been spoken. Language change happens pervasively, even in isolated speaker communities, so even if there had been some isolated enclave that cut off all contact when proto-human was still being spoken, it's vanishingly unlikely that they would have continued speaking perfectly preserved proto-human tens of thousands of years later: their enclave's language would almost certainly have gradually changed into a modern descendant, and the lack of contact with other languages would have made it appear to be a language isolate (i.e. a language with no observably related languages, such as Basque, Ainu, or Haida). That's not unlikely: in fact, if the proto-world hypothesis is correct, then that's how all language isolates did in fact form! But we have no evidence at all to suggest that a speaker community either would or could speak an unmodified form of a language for tens of thousands of years. The Sentinelese language hasn't been studied at all—it's hypothesized that it's related to the nearby Ongan languages (spoken in the same island chain as Sentinel Island) but when Onge speakers have attempted to communicate with the Sentinelese, they haven't been able to understand what's being said. (Which doesn't disprove the hypothesis: English and German spring from a common ancestor, but if an English speaker heard German for the first time, they too would have no idea what was being said!) But as I said above: just because a group is isolated, that doesn't mean that their language isn't changing just as much as others—it might just be changing in different directions!