I don’t know this specific standard, but let me suggest some general perspective.
One-way valves are intended to allow unrestricted flow in one direction and no flow at all in the opposite direction, ie an ideal 1w valve would always be open (or “open under zero pressure”) in the intended direction, and require infinite pressure to open it in the opposite one. In reality this is not practicable - to make a working 1w valve some minimal opening pressure would almost always be required; however it’s usually the design goal to keep this pressure very low. I guess that the standard authors intended to ensure that pressure is sufficiently low as to not hinder flow (in the desired direction) in most situations.
1.2cmH2O is indeed a very low pressure so it makes sense to me.
