You are correct. The slide is non-functional in the cheap prop they handed the actors for this important close-up scene. Prop 1911's ARE manufactured that DO have a functional slide, and DO eject the spent brass casing from the blank. Dwight's was not one of those. No movement, and thus no ejection of brass.

 

I thought the thumbs were on the lower half of the frame. If the thumbs are already on the moving slide and don't get hurt, that should settle that.

Please don't. You already most likely have the answer, why find out the hard way this is different? OTOH, if you DO, make sure you get someone to film it so we can see you dancing around, cursing and shaking your hand afterwards!