Having inters on meant they needed the rain. It rained 5mins later than what they needed. So they weren't far away from being perfect. If it started to rain on the outlaps they would continue and set a time on a damp track,
By the time the others went out on slicks and pitted for inters the track would be more wet and Ferrari would get 2 laps in before the others did 1. That was the gamble , it was rolling the dice and thats something they have to do now in the chamoionship.
They did it with both cars as they need a 1-2 to maximise the point gain on hamilton.
At the time I thought they had little to lose. If it dried out they could come in and get slicks, if it got worse they would be 1-2. The only outcome that could screw them was the track being drier than expected and the rain holding off for 4-5mins. Which is exactly what happened.