Note that the most important factor here is that they already were being featured on the front page. Making awesome promo art is a good idea, but if they weren't featured on the front page, this promo image wouldn't make a whole lot of difference.
If they weren't being featured, the promo art would only be seen by users that clicked through to the app's page - users would only have seen the app's icon before getting the app's landing page.
So I think featured vs not featured makes a big difference to the importance of the promo art.
They are talking about their install rate not CTR.