Join Date: Jan 2008
Location: NYC, NJ, Long Island
Programs: DL DM, 1.6MM, UA Silver, NEXUS, SkyClub & AdmiralsClub Lifetime, Bonvoy Titanium
Posts: 6,645
You don't have to book through the AMEX Travel portal . You just have to charge it to the AMEX Platinum/Centurion card.
I've booked directly with DL, AM, KL, IB, and AZ and always gotten the 5X MR points.
One caveat - I don't know what happens when you book it through an OTA since I've never done that,