Someone PM'd me wondering if this type of spoiler is effectively a "deck lid extension":
Answer: no. It's a roof extension. (Obviously you know that.) Some might call it a mini, mini Kamm back.
Like a deck lid extension, most would agree it's beneficial for reducing drag, vs. not having one, but for different reasons.
A deck lid extension helps by giving detached flow off the rear of the roof/rear window the opportunity to reattach; also a deck lid extension may promote cleaner flow separation from the rear deck.
A roof line extension - provided it continues plan & profile taper - reduces drag by reducing the size (cross sectional area) of the trailing wake where it separates from the back of the car. It may also reduce drag by reducing rear lift (vs. not having one), depending on the shape of the underlying glass/bodywork.