MacOS – turn the display off without putting the computer to sleep

There's a really simple way to test this if you have another Mac, or any other machine really.

Turn on one of the Sharing features on the Mac in question (File Sharing, Remote Login etc.) and try to connect to it from something else. If the entire machine is asleep it won't work. If just the screen is asleep it will.

I assume you're connecting to some kind of wireless network with the MacBookAir, so you could also check the list of connected wireless clients on your router. If the entire machine is asleep it should not be connected. It may still show up in the list of DHCP clients (clients that have requested an address from the router recently), but it should not be connected.

I always use this combo as well, however this does not immediately sleep my machine in ML.

I believe this is a separate issue at play here. According to Siracusa's ML review, unlike in previous OS X versions, in 10.8 your system is eligible for sleep immediately upon "user inactivity" (the inactivity delay being set in your energy saver settings) unless the currently running apps make "power assertions".

In this case, that key combo is signalling to the OS that "user inactivity" should be triggered right away. In such a case, if your particular app that's downloading stuff does not make power assertions, your system will sleep.

I have come across the same issue generally (not necessarily related to triggering the screen sleep combo). See: Mountain Lion sleeps too aggressively