In an e-mail, Shuttleworth explained to me that, "Stabilizing and maintaining the kernel is very important to Ubuntu. We have 25 or more kernel engineers at Canonical, focused on device enablement from ARM through to Xeon, and QA [Quality Assurance]. Our focus is on the user experience, making things 'Just Work,' and quality. The kernel team plays an important role in making Ubuntu so easy to use on everybody's laptops, servers and clouds, and they maintain arguably the most widely used kernels in Ubuntu releases."
Specifically, "Our teams have lead the ARM tree unification effort in Linaro, and we lead Linux kernel efforts among the vendors that ship devices with Ubuntu. We also maintain AppArmor, which is a security framework that takes the rigor of SELinux [Security Enhanced Linux] and makes it easy to use."
Shuttleworth admits that "Nevertheless, the kernel is a tiny part of the user experience of Ubuntu, and we don't lead kernel development as a particular goal. We lead with Unity, with MAAS [Metal as a Service], with Juju [Ubuntu's service management tool] and with other tools that make Linux a fantastic environment for developers, system administrators and end-users alike."
He has a point.
Sure, the Linux kernel is important. Without it nothing else could run. But, as Shuttleworth points out, Canonical does contribute a lot to the larger Linux community. In addition, Ubuntu has helped broaden Linux's audience, and Ubuntu itself is the basis for other popular Linux distributions such as Linux Mint, Peppermint OS, and Turnkey Linux. The bottom line is Ubuntu may not have contributed that much to lines of code in the Linux kernel, but it's made big contributions to Linux in a larger sense.