As far as I know, the map tiles google serves are still in mercator projection -- it's just that mercator was always designed to be projected onto a sphere, not a plane, so now it looks better.
What S2 is, is a better way of representing sets of coordinates within GIS systems, such that searching for features that are close to each other, or within a complex polygon, can be performed much quicker than with other coordinate reference systems ie. floating point latitude/longitude pairs. They're storing points on a 3D sphere (unit vectors) as easily subdivisible 64-bit integers which have a logical order which is much faster to index / iterate over nearby areas.
Makes sense for applications like google maps; "find local businesses near point X,Y" etc.