Other than war plans, religious text was pretty much the only thing in the 1500s that would have been encrypted. However war plans would be very unlikely to be disguised as a botany book, for all kinds of reasons. War plans are temporary, not something you'd dedicate that level of artistic effort and permanence to.
I mean it's theoretically possible a 1500s King might have made that book illegal, because of it's general knowledge. That's a legit point.
Sadly the radio carbon dating disproved two of my far out theories, which was, 1) The book survived from some earlier 'iteration' of life on the planet, where all plants were simply different. or 2) All planets form the same 'kind' of carbon-based life, and this book was sent/delivered to us by another planet.
Sadly, it's probably just someone's form of "art", and not even "real".
