Just to add to JOUY31's answer, in fact it is not just DL, it is the default rule on Skyteam that Skyteam elite plus status only gets you lounge access (on any member airlines) if you travel internationally on Skyteam on that flight or within 24 hours. Some airlines - notably AF (but also AZ for instance) - choose to go beyond this rule and also extend lounge acces on pure domestic trips but this is the exception rather than the rule.