When someone told about what3words, they also told me that they made money by selling shorter or more pleasant word pairings to whoever was willing to pay, but the only reference to that I could find is [0]. Having that requirement would make it much more difficult to build such a system.

Apart from that, it's basically "just" geohashes with a different (population-density based) encoding.

[0]: https://techcrunch.com/2013/07/08/what3words/