>Which part of Arch's design prevents the issue described in the grandparent post?

As a former long-time Arch user, you're correct. It's "just" a distro not unlike the biggest one. The reason Arch repos are fairly well updated and big is the relatively easy to understand PKGBUILD format and the tooling around it, which lessens friction on package management.

The difference is that arch doesn't have downstream forks of repositories. They just package the upstream versions which means the dev cycle is much tighter which in turn means they can update more frequently which means they just switch to the next major version and don't worry about breaking their special sauce forks. The downside is that there are people using old distros and stuck with old major versions who use eg. Python 2 by default.

Regardless of the explanations, as an Arch user, I think you have to at least acknowledge that Arch does not have this problem in practice. I am sure there are counter-examples but in almost all cases Arch packages are extremely up to date. If they are not, it is probably a package you are not even going to find on another distro.