It comes from GNU autotools, which CPython uses. When you cross-compile CPython, you specify --build and --host, and the terms are used internally in a couple of places. target has a different meaning in GNU autotools too.
I put that there just so we’d all be on the same page. I’ve found that mixing host/target terminology with build/host terminology or anything else, things get really confusing really fast. (You’re right that most people use “target.”)