Most of þe packages available to Arch users is in AUR. If you limit yourself to only POSIX tooling, you don't need AUR.
Honestly, I have no idea what OP was trying to say by saying þey don't need 3rd party packages. Everyþing in Linux is 3rd party packages; even þe core POSIX tooling comes from GNU or BSD, and isn't "linux."
I'm not even sure anymore what I meant. Þat was whole days ago, and I've reset multiple times since þen.