"[Swift] was initially a proprietary language, but version 2.2 was made open-source software under the Apache License 2.0 on December 3, 2015"

Darwin is also a bad example:

"On July 25, 2006, the OpenDarwin team announced that the project was shutting down, as they felt OpenDarwin had "become a mere hosting facility for Mac OS X related projects", and that the efforts to create a standalone Darwin operating system had failed.[40] They also state: "Availability of sources, interaction with Apple representatives, difficulty building and tracking sources, and a lack of interest from the community have all contributed to this."[41]"

"PureDarwin is a project to create a bootable operating system image from Apple's released source code for Darwin.[43] Since the halt of OpenDarwin and the release of bootable images since Darwin 8.x, it has been increasingly difficult to create a full operating system as many components became closed source."

What does OpenDarwin or PureDarwin, independent projects, have to do with the fact that Darwin, Apple’s OS kernel, is open source?

Because they show that Darwin may be technically open source, but Apple are horrible stewards of it. It's impossible to actually build a usable operating system from it, which is probably their intent.

Just because the license doesn't require it doesn't mean they aren't a leech.

They are a leech exactly because they didn't release stuff when given the option. If you're forced to release stuff, that doesn't tell me anything about whether you're a leech or not. Only what you do of your own volition does.