When I download that PNG file it is 10785 KB. If I re-encode it using FLIF 0.4 (to my knowledge still the best off-the-shelf lossless image compression) it is 7368 KB. That's 57% of the size of qoi, quite a lot smaller. You pay for it in encode/decode time though.
FLIF has been superseded[1] by JPEG XL, which, in my very limited testing, performed better (speed and compression-wise; libjxl) than FLIF.
Interesting though that FLIF 0.4 was released 3 days ago. I haven't checked out that new release (and won't), but the previous one wasn't great code and was a pain to use (ended up using its CLI instead of the lib). We ended up going back to PNG because of interoperability and speed. I haven't looked at libjxl's API yet.
Edit: 8605KB in 11.5s with libjxl, so it's not actually better than FLIF 0.4 (7368KB in 23.9s)
Interestingly I actually managed to shave 400kb off of that PNG by just running it through some PNG optimizers. Not great savings but were still some to be had!
