Yes, they'll be able to use DL lounge, but not "under Skyteam rules" but just because DL lounge is the lounge now officially used by AF at NRT. AF just closed their own lounge at NRT (end march) because they are now focusing TYO business in HND.
If AF still had their own lounge at NRT, because your parents are booked under JL flight number and JL is not a Skyteam partner, they wouldn't have been able to go to the DL lounge.