The existence of a way to shard searches doesn't make scaling real time search on email (hint: do some back of the envelope calculation on how much data that involves) a non-issue.
Fair enough, I too readily dismissed your drive-by objection, however non-specific and snide. Scaling real-time anything is never easy. My main point was that cost effectiveness is what keeps GMail from offering a similar level of search quality. If Greplin can get a large enough fraction of users to pay for their service, the technical challenges are surmountable.