If I understand correctly, the issue raised has actually nothing to do with Platinum Guaranteed Availability or BRG.

The 72 hour guarantee at rack rate applies to cases when a hotel is full, i.e., the hotel will actually walk another guest in order to accommodate the Platinum guest (provided conditions are met - not a blackout date, not a resort etc.).

In this case, the hotel actually has available rooms, for sale both directly and over 3rd party websites, but not via SPG.

I faintly remember such cases have been discussed here before. In any case, it's not very satisfactory from a customer perspective.