I was held up by the BKK airport takeover in 2008. My insurance policy also required some kind of statement from the airline confirming the delay. Since the news was quite well known, Eva's ticket agent who changed my flights, just initialled the new flight schedule with a statement "Flight cancelled due airport closure". That was good enough for the insurance company.

I think any reasonable explanation signed by someone in the airline is good enough for insurance purposes, so long as it's true.