The rule is enforced at the captains discretion... a UA flight may also enforce the "30 min rule" if the captain so desires, but I've really only seen it on NW and DL in my years flying in and out of DC.
...and yes it is completely ridiculous and you should write to Delta about it
Originally Posted by
indogulf
There is a DCA specific rule in place that I cannot go into detail on a public forum.
Your crew may have worded it incorrectly but they were not wrong about there being a rule.