As someone pointed out on the other thread, looking closely at the front of the car, it looks like there is some front-end damage and there was speculation that radar was broken / rendered inoperative by collision damage. If so, I'd be suprised that FSD would engage if some of the necessary sensors aren't working. Having said that, a rapidly closing radar return would trigger emergency braking... if you're reliant on the front camera, maybe not so much.
Does any one know which sensors (Cameras, Ultrasound, Radar) need to be working to engage FSD?. I wouldn't be suprised if this car has been modified, possibly to disable hands-on steering nags. Even if FSD computer vision didn't see the truck, Radar should be a cross check, if it's working, albeit that the truck body might be made from composites (Stealth Truck).