That is a link to the Skyteam benefit. The Worldperks benefit is as follows:


Guaranteed Confirmed Reservations
For our WorldPerks Platinum Elite and Gold Elite members, you’re guaranteed a confirmed reservation on most Northwest-operated flights, anywhere in the world! Simply purchase a qualifying Coach Class ticket at least 48 hours before flight departure. World Business ClassSM tickets require a 7-day advance purchase.

PLATINUM ELITE MEMBERS
Valid with purchased full Y fare or -26/-27 type Coach or full J fare World Business Class ticket*

GOLD ELITE MEMBERS
Valid with purchased full Y fare or -26/-27 type Coach ticket* (Note: -26/-27 type fares that require an advance purchase are not applicable.)

* Northwest reserves the right to limit this offer in certain circumstances where it is infeasible, such as smaller aircraft service within Asia and/or during major world events, such as but not limited to: the Olympics, major auto shows, Super Bowl, etc.

The segments I could not get were between MSP and NRT