I'll stick with my original description since that seems to be how Edelbrock uses the term. If you look at any of the manifolds that Edelbrock calls "air gap" they all have the crossover deleted and the runners are fully surrounded by air. The intakes where the valley floor is isolated but the crossover is still in place are called the RPM. So it appears that Edelbrock has defined "air gap" to mean fully isolated runners.

The big block Mopar RPM could be turned into a full "air gap" intake by eliminating the cross over and machining away the webbing on the sides of the plenum. I made one of those intakes a few years ago but it took a lot of work and probably wasn't worth the effort.