Curved fingers, flat fingers, whatever works. It's what comes out that counts, not how you make it come out. You can't be dogmatic about things like hand position.
Ah but it is a problem when it that etude sounds cloudy. Although what comes out is what counts, without how you make it come out, it probably won't come out. For instance, playing that etude with flat fingers makes it sound cloudy (for me). In order for it to be clearer, I could simply curve my fingers (while hitting the right notes!).
Pretty much two parts to piano playing right? what and how. What is pretty much the musical aspect of playing (personal interpretation, communication with the audience, etc.) and How is pretty much the technical aspect (voicing, dynamics, control, pedaling, etc.)
Sure What is what really counts, but without how, how are you going to achieve what? And if there isn't a what, what's the point of how? Both are vital aspects of piano playing.