I interviewed with TGS and two friends of mine work there. They're not "the secret Renaissance Technologies." The only commonality between them is that they're both successful quant hedge funds. They have completely different origins and tend to hire researchers from different areas of science. You might as well call Edgestream the secret RenTec since it also flies under the radar and was founded by someone from Renaissance.
This is an editorialized title which isn't endorsed by the article's content. Moreover it's from 2014. I'm not sure what sparked the sudden interest in these companies on Hacker News this week, maybe it's the new book about Simons and RenTec. But the article's pretty out of date and it doesn't give much info.
Worth noting that TGS is also several times smaller than RenTec. They only just passed 100 employees very recently. It's true they're secretive and they like to hire academics. But that describes a large number of really good quant shops. By design, you don't know who they are and they like it that way. Most don't even have websites!