Actually this is outdated info. Airlines now use the MSC (Most Significant Carrier) rule for international flights. The airline that is the most signficant carrier of your itinerary is the airline whose rules you need to follow. So if DL is the primary international carrier, it is DL's rules you follow. I have found the airlines to be really good in knowing these rules, since they are in fact US federal regulations now.
That's interesting -- I thought MSC was just an IATA Resolution, not a US federal regulation.