But in a real landing, they wouldn't want to be accurate relative to some GPS goalpost, they need to be accurate to whatever physical reality turns out to be. Reconciling expectations about were the landing location was supposed to be and where sensor input says it is must be of major importance.
That link tells us that the signals are intended to be sent with an accuracy better than 200 cm 95% of the time. Using more frequencies only really helps avoiding errors from bad/misleading reception, so it's hardly relevant in "ideal situations".
Long term stationary receivers can resolve to millimeters, but dropping rockets from orbit isn't exactly a thing that would be considered "stationary" in this context.
