Enlarged diagram on f68v2-f68v1?

[ololololo]

On the contrary, I suspect that the marginalia were added by someone who did not understand the book at all.

Or at least he thought he did...

Maybe he did not intend to sell, but to give as a present to a good scholar friend. Or to a local big shot, in the hope of receiving some benefit in return. Or maybe he meant to leave it as legacy to his heirs, or to the world.

This sounds more likely. But in this case, the book could have been sold by an heir who had previously revised it, tried to decipher it, and realized that he didn't need it.