The point was meant to be that the Units defaults set for parts, drawings, assemblies are not used as defaults for Assembly Boolean.
No matter what, the Assembly Boolean is created in Centimeters.
Even if 'Set current settings as the default settings for parts, sheet metal, and assemblies' is checked with Units/General set to Millimeters, and results of an Assembly Boolean are not saved into current assembly, the units of the resultimg assembly are always in Centimeters.