Normally I object when people use "all-time" for stats which were only kept after decades of play, but here "NBA All-Time" is the name of the series, keeping it consistent makes it easier to search.
They're saying that "NBA Leaders" in off reb started in 1973, since that's when they were officially recorded. If we had a way to figure it out, I would bet that Wilt and Russell would take the top two spots.