He defines 'vanilla' to mean 'build from upstream with little distro-specific changes'. So what does this have to do with making Python 3 the default python interpreter? I bet this symlink is not even part of the python source code; it's always a choice made by the distro maintainers.

Arch is known to be always cutting edge even if this sometimes means breaking stuff so this decision is not exactly surprising.

Removing an API that vast majority of Python programs were using (at that time), and replacing it with something incompatible is not a "little distro-specific change".

At the point where that was done, there was no formalized convention on it. Arch maintainers decided that it's preferable to default to the latest and greatest (which seems to be the general policy, and it meshes well with a bleeding-edge rolling release IMO). It was a mistake for Python specifically because of idiosyncrasies of Python 2 vs 3 story, and they have since corrected that mistake.