I was expecting this article to be about skew between HOME environment variable and getpwent(3) (usually from /etc/passed) views of the home directory.

They can be different things. Suppose your user is foobar, ordinarily homed at /home/foobar. You can write HOME=/tmp/my-test-home. Then, ~/qux will become/tmp/my-test-home instead of the usual /home/foobar/qux. That's because bash and zsh use HOME to resolve ~.

Almost. If you write ~foobar/qux, you get /home/foobar/qux again because the ~-with-username syntax looks up the getpwent home directory not the environment one.

And of course language runtimes are all schizo about whether the "user home directory" API uses HOME or the getpwent database or whatever to determine the home directory.

It's a mess, TBH. It used to be useful to temporarily bind HOME to something else to do things like create isolated test environments. Now, because of the aforementioned schizo sprinkler of randomness in the environment, you're going to have a bad time if you don't keep HOME synced to getpwent home directory.