Quite the opposite, as this text ignores the fact that Rule 21 includes unforeseeable conditions as a reason to refuse of transport. Here there was an unforeseeable condition, the need to use seats for a replacement crew because UA4600 Denver-Louisville was going to be delayed for an unknown or very long time, perhaps even cancelled, making it impossible for the UA4600 crew to fly the next morning out of Louisville as scheduled.
Except that all the rules of Rule 21 fall under the heading "safety". This was not a safety issue.
But it was a customer-service issue. They could easily have gotten the crew there on time by properly finding the market for voluntary bumps.