That "bored rich person's art project" angle makes so much sense. I read a deep dive once on how expensive the pigments and parchment were, way too pricey for some random monk. It totally fits as a flex for a noble who wanted a weird book to show off, maybe copying old medical texts to look smart.
Nah, I gotta disagree on that one. If it was just some rich guy's flex, why go through all the trouble of making a whole new writing system that no one can crack? That's a ton of work for a prank. The text seems to follow real language patterns, which is way beyond a simple art project lol. Plus a lot of the plant drawings don't match anything real, so they weren't just copying old books. Feels more like someone was trying to put real, but secret, info down. A bored noble would've just made a pretty picture book.