The grand majority of code in the Linux kernel will never be hit by booting it in a QEMU with init=/bin/shutdown. The pure line coverage of these builds is probably like 5 or 10 %.
With 16%+ lines in all 24 architectures combined, you're not going to reach more than one of them at a time.
10% is going to be an extremely happy case. If we can test 5-10% that's a great achievement. To get past that you need to start booting real hardware with specific configurations.
Note that our CI is doing way way more testing than most kernel developers do. It's not meant to exercise 100% of the kernel, just be a basic smoke test to see if we really messed something up.
From triaging reports daily from kbuild test robot aka "0day" bot, I'd say 1/3 to 1/2 of kernel commits pushed by various developers to their trees have never even been compiled (very obvious mistakes regardless of toolchain).
We get way more coverage in actually shipping these kernels in Android and ChromeOS.