If you're on a Delta-coded flight, then the Delta rules should apply.
e.g. MQM rules from:
https://www.delta.com/skymiles/about...ions/index.jsp
Delta & Delta-coded flights
(Except flights Operated by Continental Airlines or Northwest Airlines)
First, BusinessElite®, Business P, F, A, C, D, J, I, S = x 1.5
Delta-coded Flights Operated by Northwest Airlines
First, BusinessElite, Business P, F, A, C, D, J, I = x 1.5
And you're right! Those are great fares!!