Hmmm... HDR600... with 400nits display... I am not impressed...
It is like marketing red car as a blue car... they both are colors but still...
The 400 nits is the general brightness not the maximum as stated in this article. I'd assume, like most HDR 600 monitors, it can go brighter for a while in places when necessary.
I'd take this article with a pinch of salt since ViewSonic on their website say 400 nits is the typical brightness, not the maximum. Which is par for the course for these types of HDR 600 monitors.
It's kind of like runner being able to run 25mph. They might be able to do it in bursts, but they wouldn't say it's their typical running speed. It might be something like:
Speed: (typ)15mph (max 25mph)
And similarly for monitors it's like:
Brightness: (typ) 400cd/m² (maximum 700cd/m²)