The package artix-archlinux-support doesn't actually instruct you to add the regular Arch Linux [extra] repo in pacman.conf. Only the commented out testing repo [extra-testing]. Pretty minor but I would believe if you are adding Arch Linux repositories it would most likely be because you want a package from [extra] not yet on Artix Linux repos so it is weird it is not included.
For comparison, the wiki does state to add the [extra] repo so this is a very minor issue as most people would go to the wiki first to figure out how to enable Arch Linux repos, and then install the package.
https://wiki.artixlinux.org/Main/Repositories#Arch_repositories (https://wiki.artixlinux.org/Main/Repositories#Arch_repositories)
:: Running post-transaction hooks...
(1/1) Show archlinux help...
==> Add the arch repos in pacman.conf:
#[extra-testing]
#Include = /etc/pacman.d/mirrorlist-arch
#[multilib-testing]
#Include = /etc/pacman.d/mirrorlist-arch
[multilib]
Include = /etc/pacman.d/mirrorlist-arch
==> run: 'pacman-key --populate archlinux'
Hah nice find. The script has an inverted condition (https://gitea.artixlinux.org/packages/artix-archlinux-support/src/commit/5cb239cff8e473e89904c629e70b6e2a10f8e189/arch-repos-hook.script#L38) for specifically extra. I'll fix that.