Unless you think they have a "how much is the wing actually attached" sensor on the car, then they really didn't have any more data than anyone else. The fact it didn't come off in the back straight was a miracle, the fact it came off soon after was inevitable, it was absolutely the wrong call and not a gamble, it was entirely stupid. If part of the wing snapped off like, I forget who had that happen this race, but the two support parts were intact and you wanted to risk going around with lower downforce, that would be one thing, but a wing which supports HUGE huge loads under downforce that is hanging on by a thread, not a chance. They had Alonso on camera with a wing hanging off, thats the only data Ferrari had, and the instant and incredibly obvious decision is pit straight away. If one of the support struts(whatever they are called) broke but the other was completely intact, the wing wouldn't have actually been at that angle all the way across, but might have been bending/flexing at the left end. The fact that it was on that angle and clearly not in full contact with either of the struts... it was definitely going to go.
Any number of showers, safety cars and other events in the race could have gained him time back later in the race, risking the rest of the race on the first lap is mental.
Fact is it was a poor decision not to tell Alonso to limp down the backstraight and then come straight in, let alone leaving him out. It could just as easily have broken on the backstraight.