My way of thinking is: most users will just use GitHub mirrors, like they already do. Committers need the read-write repo URL. I believe the public, read-only, project-hosted repo is useful to much smaller group of people, and can be left to the other docs; development(7) is supposed to be as concise as possible.
What will end up being the official documentation, once it lands, states that gitrepo is only to be used as the push URL, whereas git.f.o is used as the fetch URL.
If I recall correctly, the idea is that commiters (and everyone else) in Europe, Asia, and elsewhere shouldn't have to fetch over extremely long links as git.f.o is on a CDN, but we can still get data from a project-controlled source of truth, rather than through GitHub.
It's not unlike how svn.f.o was accessed by everyone, and repo.f.o was where things got commited to, as noted by the old commiter's guide.
My way of thinking: development(7) should be self-sufficient and independent of any third-party service that is not part of the Project's own infrastructure and controlled directly by us, such as GitHub. Those users who prefer GitHub for some reason should ideally be reading its documentation, not ours. On the other hand, I guess it makes sense to mention it as it is a popular community marketplace which most users are probably familiar with.