It takes a few seconds for a human to move their foot from a pedal to the other for a sudden stop to happen
And with a Tesla, once you lift your foot off the accelerator pedal, the car slows down
It doesn’t seem like it did, but rather performed a sudden stop
Other cars including the one filming kept rolling a bit before it stopped.
Which leads me to believe this was actually on FSD, though there’s no evidence other than if the driver said so, but looking at how it stopped
It was most likely not n FSD.