They just "acquired" Sparrow because it was an app that used their infra-structure, but bypassed Google Ads completely (showing their own ads in the free version instead).
I don't that will happen any time soon. If they do produce an app, they would cut revenues coming from advertising so I don't think it is in their interest. On mobile, the situation is different as mobile users expect to have features such as notifications, and offline browsing (I know the latter can be done using HTML 5 but users' mind is not yet at opening a browser when they know they are offline).
Read what s/he said. There is lost ad revenue on a desktop mail client, so the parent is saying that it makes sense for Google to push the user to the web client. This doesn't work on mobile because the user has certain expectations of how mail works on mobile.
