Uh, so the program is almost as fast as libdeflate at compressing at a given level. But the post makes absolutely no attempt at establishing that a given level compresses as well as the corresponding level in libdeflate. In the linked post there is a graph detailing that libdeflate does indeed compress quite a lot better than the original lean-zip at the same level.

Without data on compression levels it is hard to make any useful conclusions.