Except it's BSD licensed and not necessary to share the code.

True. But Darwin is still not a thing :) There is no real community, bug tracker, etc. My point that it's just an Apple code dump remains valid, I think.

Open source doesn't necessarily mean open development. Companies like Apple embrace the former without the latter. It is perfectly valid to share your source code openly but not allow outsiders to do development. On a lesser scale, SQLite is another example of open source without allowing open development. You can't really contribute code to SQLite even if you wanted to. Do you think SQLite is not a thing? Heck, even RMS used to be super protective of Emacs.

In the area of kernel development, maybe, but Apple develops WebKit, Swift and some other projects in the open.

I did not say it was not open source. I said it's just a code dump, and so far you have not really proved me wrong.

There is not even a proper repository with version history, as far as I can tell. Just this stuff: https://github.com/apple/darwin-xnu/commits/master

That's literally a code dump.

Apple has in the past provided a fully bootable Darwin operating system -- no Gui, but the OS worked just fine.

Your argument is pedantic where pedantry isn't warranted or necessary -- Darwin is a BSD and is BSD licensed, Apple doesn't have to share anything, but they do.