A Swift program for a particular distribution will dynamically link some system libraries from the distro, and these libs might change on every distro update. They mention in the post that dynamic linking can cause versioning issues.

> Say my Linux distribution distributes some Swift runtime

That runtime would need to be compatible with the Swift program. Nowadays that’s not a big issue due to ABI stability (https://www.swift.org/blog/abi-stability-and-apple/), but this would close the door on new features that need runtime support (need an OS update then, or the OS must come with multiple runtimes).