What happens if you try to launch a drone when the wings are folded? Is the DJI Smart Controller / app smart enough to know that they're holded and note actually start the motors?

Did you mean the arms? If so, no, the controller will not stop the motors from starting but you will get an error about motors being blocked and it will quickly turn off power to them.

If you meant the props, the controller does not know that the props are still folded. The props  will unfold due to centrifugal force but that is not recommended because  there will be a lot of vibration while that happens and damage could  occur. I mistakenly did that once...never again.

If the props aren't open on the Air2S you may also see a warning about the motors which is new to the Air2S I believe.