Is this an advertisement for S2? there is no description of alternatives, of how the choice was made, no comparison to any other system or provider, ...
Redis has geo search for example, why not using it?
geo-indexing is different from geo-sharding, search indexes normally have geo-indexing, but it is still one big index handles every single search. The alternative(geohash) is mentioned in article, which has high distortion around earth poles.