That is the exact language on the T&C and literally it does mean what you mention. However, so far it has not been applied at its face value and I guess it should be read in conjunction with the other provisions as meaning that, within the same booking conditions (cancellation, deposit, etc.), one has to book the lowest available rate on spg.com — for instance, where there is availability for a Members rate plan and a Preferred Rate, one has to book the former to brg against an OTA cancellable rate.