That site is misleading (it's not an official site), as you CAN use it for flying into the US without a passport, just depends on the airport.Yep. Directly from the application site...
PROOF OF IDENTITY
If you do not have a passport, you can get still a NEXUS Card, though it is not recommended because you will only be able to use it for driving across the border, not flying. The most common proof of identity that is not a passport is a Driver’s License.