Every kernel subsystem has its own testsuite. Running all of them would requires hundreds of different pieces of hardware, so it's not really possible for a single release manager to do so.
For Linus's releases this is easily solved by slowing down progressively the pace of development towards a release, so that cross-subsystem issues where maintainer A breaks maintainer B's subsystem become progressively less likely over the two months of the release cycle.
For stable releases this is much harder to do because of the short cycle. The stable branches in the end are a mostly automated collection of patches based on both maintainer input and the output of a machine learning model. The quality of stable branches is generally pretty good, or screwups such as this one would not make a headline; but that's more a result of discipline of mainline kernel development, rather than a virtue of the stable kernel release process.
Yes, I agree that _this_ issue could have been found. But the parent was talking more in general of "doesn't the kernel have a test suite", and both hardware-dependent (drivers, profiling, virtualization, etc.) and hardware-independent (filesystem, networking, etc.) aspects of the kernel are distributed across multiple testsuites.
The stable kernels pre-release queue is posted periodically to the mailing list and subsystem maintainers _could_ run it through their tests, but honestly I don't believe that many do. Personally I prefer to err on the other side; unless something was explicitly chosen for stable kernel inclusion and applies perfectly, I ask the stable kernel maintainers to not bother include the commit. This approach also has disadvantages of course, so they still run their machine learning thingy and I approve/reject each commit that the bot flags for inclusion.
