We should perhaps note that while the scale of Froome's acceleration while not increasing heart rate or getting out of the saddle is more extreme and remarkable in comparison, Contador had done the exact same type of thing, a seated attack without really attacking, to drop Fränk Schleck and António Colóm in 2009 at Paris-Nice.
Here's the video from Montagne de la Lure. At around 20:30 Contador is riding on the front of the trio, and a few seconds later, with barely even a change in cadence (which is admittedly a significant difference from Froome's Ventoux move with the kick into egg-whisk cadence), he just rides away from the other two opening a noticeable gap quickly. However, the TV cameras move away from focusing an Alberto to look at Fränk and Toni for a few seconds then when they go back to him he's moving so there's no way of really seeing how it transpired.
Again, nothing that can be considered even remotely proof of anything, and because we can't actually see Contador on-screen at the moment of the attack, difficult to draw any conclusions from - but it is evidence that Froome's seated beast mode attack, bizarre as it was, and regardless of how much suspicion and/or ridicule it has attracted, is not as totally unique and unprecedented as it can be characterized as.