But why would anyone use Omarchy (based on Arch) and not use Gentoo directly? Not a separate project based on Gentoo, but a Gentoo Reference System [1]?

Gentoo already has all dev tools installed - they're an indispensable part of the package manager.

[1] https://wiki.gentoo.org/wiki/Project:RelEng_GRS

Why bring up Gentoo at all?