Is there a reason why Premium wouldn't give me aisle?
Premium gives you access to the Aisle and the Emerald Reserve spots for Premium, FullSize, Midsize, Compact & Economy. Don't mind the app not displaying it.
BTW, careful when choosing premium in that screenshot as it will take you to the fullsize pricing if your a executive member. The fullsize pricing looks to be quite a bit higher. Choose luxury if available or call National to book the reservation. They can force the Premium price to lock out the Executive "one class lower" benefit.