As someone who doesn't understand much of what's going on here, what resources would you suggest I study to improve?
I've found it very accessible. Largely, the way that they teach is to describe a system based on a set of assumptions, then slowly relax each of the assumptions one by one until you reach an example of a real system. That style of teaching really works for me.
Uninterruptible process sleep is covered in section 22.3. Threads, in chapters 29–33. Signals, chapters 20–22. The proc file system in section 12.1. Memory mappings, chapters 49–50.
Uninterruptible sleep, signals, memory mappings etc have not changed fundamentally.
Namespaces and cgroups are new concepts in Linux, they won't be covered. IIRC user namespaces appeared in 3.14. New cgroup hierarchy got somehow usable much later than that.
The new concepts are needed for containers. Not at all relevant for the original article here.