Could it be an unintended consequence of a sloppy wording? i.e. it can't be used for "a. the actual aerodrome QNH from an approved source" (for the reasons above) but should be covered by "b. the forecast terminal QNH" even though it isn't a forecast. Maybe someone simply forgot a METAR isn't a forecast when writing the rule?
Yeah possibly. It seems to be pretty confusing to most of the community so I've emailed the BoM. Will post their response here.