That is, you probably want to design PDF output. Some users prefer to do any design work whatsoever using web technology. Therefore, PDF needs to be generated from HTML.
In fact, he's not even aiming to design a professional-looking PDF document, just a throwaway printout for proofreaders who don't know how to parse Markdown.
This rendered HTML could be converted to PDF and printed or the self-rendered HTML itself could be printed directly.
It also looks like OP's book was split into several Markdown files, one for each chapter. So he would have needed some sort of build script anyway if he wanted to use texme on the combined document. He would also have needed more than a single line of code in the header, since he wanted some custom styling for blockquotes and code snippets.
I'd probably use Asciidoctor which gives more flexibility on the markup side already, but if this process works for them, why not use it.
True.
> and is unwilling to learn how to design anything if it is not web technology.
False and does not follow.