Yea it's sorta dumb in hindsight but In a panic I get it. A smaller sedan in neutral can be pushed by hand so he probably panicked and tried the same lul.

I’m guessing they’re probably familiar with large trucks so they knew what had to be done.

I know almost nothing about large trucks, but would assume I could just mash the brake pedal (probably with my hands because its faster than trying to climb into the occupied seat), and bring the truck to a stop. That would buy enough time for someone with large truck knowledge to secure it properly.