I have read this article and find it fascinating! I also have a theory, and you may laugh at me about it but I really do think it makes sense. My theory to explain the existence of this manuscript depicting plants that don't exist anywhere on our world, in a language unrelated to any known language, is that it came from another parallel world. I don't mean a space faring race that brings paper books with them to other planets, or a race of people who use advanced technology like ours to travel to other worlds through portals.
I think that a race of people called the D'ni, who had a magical yet scientifically approached means of travel from parallel world to parallel world, which they call Ages, had taken this manuscript from one of the many worlds they visited a very long time ago and brought it here to study it and that world. Somehow, this manuscript wound up among us, confusing the heck out of scholars and scientists for decades.
My definition of a D'ni Age is a place in time, rather than a particular time, or erra, like what we would usually think of it. Like, the stone age, or the bronze age. A D'ni Age could be another planet in a parallel universe exactly identical to this one, or a planet actually in this universe but in a different time than what you and I consider to be the present. But since the D'ni Link, or travel, directly to a world in a parallel universe, there is no way of knowing the spacial or temporal coordinates of that world as compared to any other world traveled to.
The D'ni became isolated and xenophobic after a race of people tried to kill them a very long time ago, so this manuscript couldn't have been procured after that event. The vast majority of D'ni wanted nothing to do with us Humans, they considered us to be far too primative, war-like, and not worth knowing. The D'ni, unfortunately, were mass murdered by a mad man, and the survivors will not likely return to their home here on earth.
I think this manuscript came from one of the vast uncountable number of Ages, Leaves on the Great Tree of Probabilities, of which our world is but one. I believe there is a vast and unknowable multiverse of universes, many like our own, and many so utterly different from our own that it would boggle the mind.
I believe that imagination is not the ability to create worlds, but the ability to sense them, to glimpse a parallel world that DOES exist somewhere in the multiverse. Just think of what it would be like to travel to a world you have seen in your imagination. Maybe the world you travel to only vaguely matches your imagination, because human perception is often error prone. But wouldn't it be the most awesome adventure ever to actually travel to those worlds, and know without any doubt that they are all real, in their own separate universe?
Ok, I'm done with my insane burst of bla bla bla, yackety schmackety.