I'm not sure how. I think the widespread conception is that working with emacs-devel is more difficult than working with many, many other free- and open-source projects, and I think that conception is correct. Compared to "fork the project and submit a pull request" or "publish a package on melpa", following emacs' patch guidelines is harder. Emacs requires additional paperwork. Updating the documentation (NEWS, Changelog, the info manuals) is harder and involves tools, tech, and practices that are unfamiliar to most potential developers. Using debbugs has improved a lot in the past few years, but is still a pain-point for many. Following the emacs-devel mailing list is, let's say, not a welcoming experience for everyone. That doesn't even touch on the difficulties of interacting with the core.

I think that the end result of emacs' processes is high-quality code that runs on many more systems than the vast majority of software, but I don't think most new developers are shying away from working on emacs because of the lack or quality of the documentation.

~Chad