The best guess is the insurance policies refer to unintentional ignition while SpaceX itself called what happened "an anomaly".
Mr. Musk described the event as a 'fast fire' not an explosion.
Following up on last week's "A Bad Day For Elon Musk and His Insurers (TSLA; SCTY)" wherein it was pointed out that the launch insurance would not kick in and it would probably fall to a marine cargo