Skimming the code, it seems to use bitwise and with a mask if the hash size is a power of two, which has the same caveats as described for Dinkumware (it's fast, but if your hash is poor it can have awful results), and uses modulo if not which has all the issues the article describes. This is encapsulated in a single file [1] so it looks like it'd be easy to improve.

[1] https://github.com/Tessil/robin-map/blob/master/include/tsl/...