The "statistics textbook" part is especially confusing to me -- who is the audience for this, and why is it even here?
He mentions at the beginning of the probability chapter:
"Readers are assumed to have some basic notion of the concepts associated with probabilities, and to have encountered the idea of probability in the form of likelihood of an event occurring"
This doesn't give us any insight into what background is expected here -- do I need to have taken a measure theory based probability course? Or would a calculus-based course suffice? What about a course for non-STEM majors? The word "basic" means different things to different people, and is generally an anti-pattern in mathematical writing. It's meaningless at best, and gatekeeping at worst.
Then he launches into this downright strange rabbit hole about mental models. None of this discussion in this chapter feels properly motivated, the writing is disjointed, and it reads like an unedited, non-proofread stream of consciousness. In particular, the exposition gets too distracted by its own examples.
Don't get me wrong -- many of the paragraphs in isolation are interesting, but taken together it's excruciating to read. It's an impressive project overall, and certainly ambitious, but it falls down due to the writer's lack of empathy for the reader. I suspect the situation would be better if they reduced the scope of the project and kept the writing more focused.