A very plausible argument I read a while back was: it is a manual of exercise and herbology and related things. The symbols in the book aren't in any language. They are like rough music notation; XXX, YY, XX etc. They are custom for the client of whoever wrote it, meaning "stretch, stretch..., bend, bend..." or something similar.

The theory isn't popular because it doesn't reveal anything but once you look at the book it seems likely. The notably thing is the symbols don't have the variation of a language - they are less complex and more locally repetitive.