The article says

Firefox on Linux to date has focused on the Video Acceleration API (VA-API) that works well for some vendors but lacks official support from NVIDIA unless using the independent NVIDIA-VAAPI driver project that layers VA-API on NVDEC. There are also various embedded drivers and more not supporting VA-API

So it’s basically just because the existing video decoder API in Linux doesn’t have support for various hardware devices for various reasons. But those devices still have Vulkan support. Not sure why Vulkan support for those devices wasn’t just added to VA-API, who knows.