That's just for heavy trucks. Makes sense since brakes alone don't stop a freewheeling truck so well. That's why truckers are told to descend in lower gear, stop check brakes before descending, and most run engine brakes. Rolling free down a hill in a heavy truck is dumb truckin'. Really no good reason to unless you have a mechanical failure and need to drift off the road to a safe place.
Jack Benny didn't have an automatic in his car, but that cheap skate would coast his car (rather Rochester did) down hills to save on gas (on his tv show).
Another celebrity coasting downhill... and not supposed to hit the brakes!
https://www.youtube.com/watch?v=-uO1gXmMkKY