Why did he not flatter the F14T last race, then?
Unlike a taxi driver, it's the race driver's job to get every ounce of performance out of the car.
Maybe Ricciardo flattered the RBR, too and got every ounce out of it - to the point of pissing off Vettel.
Or, maybe, the F14T just got a little bit better and on a track that suits it better?
I think in terms of performance, Alonso's Ferrari and Riccardo's RBR were pretty closely matched, with a little swing in performance in different phases of the race. Of course Alonso couldn't drive above the theoretical limits of the car and 'flattered' is just figurative. When the performance of 2 cars are closely matched, the driver makes the difference. In Alonso's case, not only driving fast but conserving his tires, making all the right decisions like not wasting any time or tire life at all in defending against a quicker Rosberg. I'd imagine many drivers to actually put up a fight in that situation and end up losing time and/or destroying their tires. He knew Riccardo had fresher tires so he conserved them.
At least that is my take on it.