a demonstrably false assertion --- sending diffs against unrolled tarballs was the method used to attain most of the Linux kernel in the early days
Please, do not compare apples and oranges.
"In the early days", the Linux kernel did not have that much code nor contributors, so working with diffs was fine (though still somewhat painful when merging stuff.)
Try to run the Linux Kernel development without a decent VCS and you will most likely fail due to the complexity of code, contributors and also roles.
This said, I don't think x2go is such a high traffic project. A VCS may still be useful, as the Developers are always complaining (and so are some users) about how much time they spent on the project and I foresee even greater qq, when using diffs only, as a merge conflict is likely to take some time to get resolved if several users are working on the same file.
Best regards,
Mihai