They talk about patch submission in the downloads page git subsection, and the documentation page welcomes contributions in the first paragraph. I can't speak

They talk about patch submission in the downloads page git subsection, and the documentation page welcomes contributions in the first paragraph. I can't speak for the Cairo project, but some projects prefer there to be a slight barrier to patch submissions to improve the signal:noise ratio. Anyone vested in a well-formed patch should be motivated enough to send an email to either a mailing list or any of the developers found in the git commit history. Reviewing patches takes significant time, so it can make sense to require the submitter to invest a bit of time before consuming yours as maintainer.