The title is misinformation. It should be “someone made a RAT downloadable on Arch Linux” because the Arch User Repository (AUR) ≠ official repository. From the Arch wiki:

The Arch User Repository (AUR) is a community-driven repository for Arch Linux users. It contains package descriptions (PKGBUILDs) that allow you to compile a package from source with makepkg and then install it via pacman. The AUR was created to organize and share new packages from the community and to help expedite popular packages’ inclusion into the extra repository. This document explains how users can access and utilize the AUR.

A good number of new packages that enter the official repositories start in the AUR. In the AUR, users are able to contribute their own package builds (PKGBUILD and related files). The AUR community has the ability to vote for packages in the AUR. If a package becomes popular enough — provided it has a compatible license and good packaging technique — it may be entered into the extra repository (directly accessible by pacman or from the Arch build system).

Warning: AUR packages are user-produced content. These PKGBUILDs are completely unofficial and have not been thoroughly vetted. Any use of the provided files is at your own risk.