What am I missing here? Why can't, as others have mentioned in other HN comments elsewhere, the OOM killer just get invoked when there's less than X amount of RAM left, and kill the highest-offending process? In my case, I would prefer that to anything else. Why does this page or that page matter?

There's no reason the OOM killer can't be made more aggressive, and there are user-space implementations of that behavior. I use the "Early OOM Daemon"[0], which is packaged in Debian. I had problems with my system locking up under memory pressure before, but so far earlyoom has always managed to kill processes early enough to prevent this.

[0] https://github.com/rfjakob/earlyoom