Agreed; Emacs is the gold standard for documentation. It comes with a reference manual (400k words), an Emacs Lisp reference (600k words), _and_ 64 other manuals for individual Emacs modes or features including one just for Calc mode (250k words), a manual just for Org mode (130k words), one for Gnus (180k words) etc. All told it adds up to about 2.6 million words of documentation.
Still, another manual written from a different perspective probably won’t hurt anything.
One reasons Prot himself was able to become a bonafide Emacs Guru in just a few years is because he's the kind of person who reads manuals. He speaks highly of the included docs, and often credits them for his knowledge.
And he wasn’t even a programmer by trade. AFAIK he used to work in Brussels at the European Commission, he started using Emacs for his writing and taught himself Emacs Lisp as a first programming language, probably by diving deep into the Emacs manual.
Or the official tutorial: https://www.gnu.org/software/emacs/manual/html_mono/eintr.ht... (which to be clear I haven't read, but have heard nice things about).
Of all the things for which emacs can be criticised, documentation rigor is really not one.