[vannevar]

Flown twice is still pre-existing---the vehicle had been in development years before the award. If your point is that the Falcon was pre-existing and it still took a long time to get to crewed flight, that's true. But note that the adage doesn't say the launch vehicle is the only source of delay, only that it is generally the most significant one.

A better counterexample would be a crewed program that developed its own launch vehicle and still completed on time and within budget. If you treat the Falcon 9's development time as part of the overall project (which you should if you're maintaining it's a new vehicle per the adage), then you're looking at something like 15 years just to get a crew to orbit. Which is consistent with the rule.