A lot of these packages have Arch-specific modifications. For example, filesystem doesn't even have a non-Arch upstream as it defines the filesystem layout. That PKGBUILD and everything it depends on is the Arch source. Distributions are defined by which packages they include.
There is even a sentence in README.md that makes it explicit:
The source files in this repo are for historical reference and will be kept static, so please don’t send Pull Requests suggesting any modifications to the source files […]