That issue is about supporting Vulkan on macOS via MoltenVK, but mpv already supports Vulkan on Windows and Linux.
The problem that I think the parent post is referring to is that mpv 0.28.0, which introduced Vulkan support, also introduced a hard dependency on FFmpeg APIs that haven't been released until now (4.0). Linux distros prefer to use stable versions of packages, so most of them have been packaging FFmpeg 3.x and mpv 0.27.0. They can only upgrade to mpv 0.28.0 (with Vulkan support) now that FFmpeg 4.0 has been released.
The problem that I think the parent post is referring to is that mpv 0.28.0, which introduced Vulkan support, also introduced a hard dependency on FFmpeg APIs that haven't been released until now (4.0). Linux distros prefer to use stable versions of packages, so most of them have been packaging FFmpeg 3.x and mpv 0.27.0. They can only upgrade to mpv 0.28.0 (with Vulkan support) now that FFmpeg 4.0 has been released.