From the 3rd-hand report, it sounds like the announcement erroneously requested passengers to remain seated until the honor guard had departed--rather than the normal request to wait until the hearse departed from the tarmac.

So, the door was opened, the honor guard got up from their seats and exited the plane, and the passengers and crew got up and left after them--just as they had been requested to do.

This is a completely understandable misunderstanding.