This fundamentally goes against the Unix philosophy though which is to provide small well-defined parts from which you can construct a complete solution from.
If you need a complex manual for a complex program, something is wrong.
What does this even mean? The name GNU itself is a joke.
I mean, the project wants to create an operating system that looks like UNIX, acts like UNIX, smells like UNIX, but from scratch with appropriate license that allows usage and access to source to anyone - so that the project doesn't get into legal trouble from whoever actually owns UNIX?
So drunk Stallman in 1981 says: "hik, let's call this, hik operhikating system GNU, hik, because it's not UNIX hik, but it sure looks like one, hik, but it's not, hik, but it kinda is hik, but it's not theirs hik it's everyone's hik". That's how I like to imagine it happened.
But we have other hypertext formats available. It'd be better if GNU started using some format that everybody already usees and knows how to write.