I think that was a precaution. The malicious build script ran during the build, but the backdoor itself was most likely not included in the resuling package as it checked for specific packaging systems.

https://www.openwall.com/lists/oss-security/2024/03/29/22

No, read the link you posted:

Arch does not directly link openssh to liblzma, and thus this attack vector is not possible. You can confirm this by issuing the following command:

ldd "$(command -v sshd)"

However, out of an abundance of caution, we advise users to remove the malicious code from their system by upgrading either way.