Can you actually recompile your kernel on Mac? And its functional?

I'll bet this isn't even the source code Apple uses, but rather they have a private fork with extra patches (similar to how Microsoft publishes OSS VS Code, but then uses their own proprietary version for releases).

Yes. However, many kernel modules needed to boot on a modern macOS machine are proprietary.

If an os is partially oss but functionally closed, what do you call that?

If I run windows with apache web server, is windows now open source?

rMS is a zealot, but it doesn't mean he's wrong...

You can, or at least could 5 years ago or so (and I bet you still could now).

However, it involved finding some random blog post on how to do it, and then even once I got it up and running there were some issues like the fans being pegged at 100%. Still, it was mostly functional.

I can speak to that via scuttlebutt from ex-apple friends. Apparently the intel power management code running in macos includes proprietary code from intel that they weren't comfortable opensourcing, for weird reasons.

There was an attempt to make a complete FOSS OS on top of that - PureDarwin[1][2]. It looks dead now though.

[1] http://www.puredarwin.org/

[2] https://github.com/puredarwin