is this a google proxy?
It does index some stuff that appear to indicate it's backed by google, you're getting Google Books results for example. They're also getting GitHub results, which you'll never get if you crawl on your own.
On the other hand there's a heavy skew toward publicly available datasets (wikipedia, stack overflow, reddit), which might indicate they're mixing and matching a bit with publicly available data. Could be wrong though.
Any new takes on search is ultimately a neat thing. I do think it's sadly under-explored design-wise. Google has been so dominant that nobody seems to think search can be anything other than a white page with a plain text input.
> At present, the content farm is flooded, and only sites with good experience are included in the whitelist.
https://github.com/txthinking/soso/
Based on that, my interpretation would be that it's an opinionated search engine that indexes a limited set of sites that it considers to be "good", ignoring everything else.