I believe the GP is talking about the software macOS ships with, not macOS itself.

macOS ships with some GPL2 software, along with its BSD/MIT/Apache-licensed Darwin layer. (The GP is presumably implying that having this GPL2ed software "built-in", helped macOS entrench itself as a useful POSIX for developers — at least before tools like MacPorts/Homebrew came along to make acquiring this type of software "after-market" simple.)

For any GPL2 software macOS makes use of that then transitions to GPL3 licensing, Apple can't actually adopt the GPL3ed version (exactly because of the "runs anywhere" clause in the GPL3) and so instead, macOS keeps the old, GPL2ed version of the software around, left to rot. Eventually, if it's something critical, Apple removes their dependency on said software, replacing it with something that's not GPLed.

This is the story of Bash on macOS: it was regularly updated for as long as it was GPL2-licensed; then it got "stuck" on the last GPL2 version when GNU transitioned Bash to GPL3 licensing. The Bash on macOS remained on that old version for the longest time, before finally being swapped out for Zsh in Catalina.

also they were technically in violation of the GPL with bash. They made some modifications and did not release them all. (try to compile bash without rootless.h)

Presumably, they are on https://opensource.apple.com/source/bash/bash-118.40.2/ although sources for the latest macOS are usually not released in a timely manner (no Big Sur at the moment, for instance)

rootless.h has never been made available to the public as far as I know.

I just found that the new dump of sources for 11.0.1 has a version of bash that doesn't #include <rootless.h>

that's odd... what does Apple have to win out of a GPL violation for this?

Nobody has forced them to release it, so until then they get to keep some magic to themselves, presumably.

In the beginning the packaged a bunch of stuff, both mit/bsd and gpl. At some point they stopped gpl software. The darwin stuff is published but not everything. The code shared has become less over time even as apple has grown enormously.

They didn't stop shipping _GPLv2_ software. They just never shipped GPLv3 software.

Emacs is gone IIRC

Parts of Darwin are APSL.