I thought Arch was the only rolling distro that doesn't have the backdoor. Its sshd is not linked with liblzma, and even if it were, they compile xz directly from git so they wouldn't have gotten the backdoor anyway.
Yes, this sshd attack vector isn't possible. However, they haven't decomposed the exploit and we don't know the extent of the attack. The reporter of the issue just scratched the surface. If you are using Arch, you should run pacman right now to downgrade.
They actually have an upgrade fix for it, at least for the known parts of it. Doing a standard system upgrade will replace the xz package with one with the known backdoor removed.
The extent of the exploit is still being analyzed so I would update and keep your eye on the news. If you don't need your computer you could always power down.