SDR panels are marketed by peak brightness, not sustained. Flash/local is peak. 400 > 350 and, in reality, most SDR panels don't even reach 350.
DisplayHDR 400 establishes a specific requirement for contrast that SDR lacks ergo, it's an improvement over SDR.
Very, very few SDR panels meet or exceed 400 cd/m2.
I'm not saying DisplayHDR 400 isn't a minor improvement over SDR because it is minor. The point is certified DisplayHDR 400 displays should have better picture quality than non-certified panels.