How on earth is distributing common Unix tools for MacOS that aren't included in the base OS not "using the underlying UNIX". This is exactly what Unix was designed to enable. Apple understand and values this, which is why they have submitted patches for these projects.
I could quibble with the "Most of" part, most of it is stuff you don't get in MacOS at all, but that aside so what?
This isn't really about alternative ecosystems, it's about complementary ecosystems. There are a lot of people that use MacOS desktops alongside Linux or other Unix machines. For these people having a common set of tools that work the same, so you can use the same command lines and scripts across multiple platforms, is incredibly useful.
What do you think people use Homebrew for? brew list gives me mailhog, mysql, postgresql, newer python, newer ruby, macvim, node, redis, ... these aren't IN the "UNIX provided in the macOS box".
As if some self-imposed UNIX/POSIX austerity, making do with the basic (and old) UNIX userland that comes with macOS (or other platforms), is something to be lauded?
(As opposed to just an example of someone making do with the little they need, where others' mileage may vary?)
Or is needing some of the tons of programs that don't come with "Apple platforms and their UNIX" (e.g. some random stuff I use: gnuplot, ripgrep, redis, postgres, jq, graphviz, and tons of different things others might want) somehow problematic?
Not even sure where Apple platforms and UNIX come into play as something to be contrasted to "replacements for GNU/Linux".
One of the benefits of macOS is precisely that as a UNIX it can run all kinds of UNIX tools, not just the basic POSIX utils, but close to everything available in a Linux/FreeBSD/etc package manager...
On the contrary, I use macOS for what it is and the value of its development stack, not as a pretty replacement for GNU/Linux, for that I already have my Asus netbook.