I don't see the sensor he is pointing to. The only system I'm familiar with is a 6x4 inch or so rectangular thing with a medium black plastic cover. There may be other systems out there.
The trucker set it off, if there is one, not the other car. They usually don't activate under 15 MPH or so, otherwise they would drive you nuts in stop and go traffic. It looks like the trucker kept his speed up, assuming the cam car would start moving again. (and it didn't)
The one I'm familiar with would just flash and beep first to give you some warning, not go into braking mode unless you continued to get closer.
There's another major manufacturer system I haven't run into, yet.
Usually they don't send an 'event report' unless the thing goes in full panic stop mode.
Maybe 'friendly trucker' has been setting off his too many times recently?
I don't see one of the commercial camera systems in the truck, which would have also sent in the video 10-20 seconds before the event, and 10-20 seconds after.
That type of tanker hauls dry products, like flour, salt, etc. He was probably late getting to the twinkie factory.