There are many quite loud alerts when FSD is active in subpar circumstances about how it is degraded, and the car will slow down. That video was pretty foggy, I'd say the dude wasn't paying attention.
I came up on a train Sunday evening in the dark, which I hadn't had happen in FSD, so I decided to just hit the brakes. It saw the crossing arms as blinking stoplights, probably wouldn't have stopped?
Either way that dude was definitely not paying attention.