HDR is about, well, high dynamic range images, usually expressed with at least 10 bits of precision (although it can also be float, etc), and often, but not always encoding scene-referred data instead of image-referred data (originally it was supposed to only encode scene-referred data, but then other competing formats ignored that). It has nothing to do with the gamut and with the color primaries, although in practice HDR images use a large color space.
But you can absolutely have an SDR image encoded using a large color space. So I am not sure why the author talks about color primaries when it tries to justify HDR… I still don’t know what kind of HDR images this new PNG variant can encode.