From the white paper:

> Despite this, the Linux Kernel is lagging behind in implementing some modern mitigations in its default configuration. Both stack canaries - which protect against stack overflows, and KASLR (kernel address space layout randomization) are lacking in most devices running Linux today

It seems that they opted not to try to bypass stack canaries, probably because of the number of Android devices running old versions of Linux.

It seems inaccurate for them to categorize this as a problem with kernel itself, however. The kernel itself isn't "lagging behind" if mobile/embedded devices won't update to never versions containing newer mitigation techniques.