There are patches required. Many GNU utilities are very close to the UNIX spec, but not quite the UNIX spec - including glibc. But making a Linux distro that is UNIX certified would likely make it a worse Linux distro for most people since it would be less compatible with what everyone is assuming for a Linux distro. A lot of the differences are subtle edge cases, but do you really want that in your distro?

It's not just about going through a song-and-dance. It's about making an OS that has different behavior - often very tiny differences, but differences that would make the distro worse for most users.