this fully 3D-printed book turns its own G-code into raised lettering

It’s called a Quine

Manual contains only 2.5 percent of its own G-code in its first version.

that carries part of the machine code used to fabricate its own body.

Seems like just a clickbait headline. It could easily contain all of the gcode for printing the book itself and not the part for printing the letters on the page.

But it might be possible to have gcode that (completely) prints its own gcode. Regular programming languages can do that, there’s a whole fun community around cool puzzles that use that. Obviously, this project does not do that.