Statistical compression works by learning statistics about the data as compression progresses. This is a(n admittedly very good) implementation of that, but the idea of learning about the data is as old as time (or computers, whichever came last). Pretty much all compressors do this, wither by building a huffmann tree on blocks and shipping the tree allowing the data to be decoded, or by incrementally updating statistics after each token, and using the updated statistics for the next one.