Also why when on a macOS system I never felt the urge to install Homebrew or something similar, it is just another UNIX flavour.
It's interesting to say that it's "just another UNIX flavour" when the most common flavours today all have some form of package management.
Two Linux distros, both derived from CentOS Linux, have also held certification in the past.
Whether an OS is a UNIX™ or not is nothing to do with whether it is based on AT&T code and has not been since Novell bought AT&T Unix Labs and donated the trademark to the Open Group in 1993. For three decades, whether it's a Unix or not depends solely on whether it passes Unix compatibility testing.
Linux is a Unix and so is macOS.