My reading of it is that it was the Treasury who used it and the Select Committee were questioning why, as it was rarely used, but also pointing out that the demand scenario adopted happened to be the one that gave the best result for Heathrow.
"Of the four investment measures used to evaluate the proposals, only this seldom-used net public value measure presents a clear case for a third runway at Heathrow, Mr Tyrie said."
Isn't he suggesting that the method was only used because it gave the result the government wanted?