I think it's been mentioned in the original XAddress post, but the problem is not necessarily mapping a lat/long pair to a human-memorable address.

The way current addresses have worked (mostly) is by associating with landmarks. Large street names are known by lots of locals. Smaller streets are associated with larger streets by proximity (second left on Large Street). Numbers help place an address at the beginning, middle or end of a street, on the left or right.

Grid cities have an added advantage of encoding approximate location in the street name directly.

XAddress and What3Words solve a problem for the 4bn without street addresses, but they do not help locate things or navigate to them without being online.