PNG compression is an absolute joke so that's not worth anything - JPEG-XL and WebP Lossless beat PNG by a longshot. PNG also requires intense optimization to be truly compressed, and I'm not sure to what extent they tried to optimize their PNG results. As an example, the lossless comparison chart I uploaded was 380KB initially, and I was able to optimize it to 280KB using OxiPNG. The PNGs in the lossless comparison chart were properly optimized.
Beating FLAC is interesting, but FLAC compression hasn't really been updated in a long time TMK - I wonder if there's traditional gains left on the table with modern compression techniques?
A copy of the original paper can be found here.