zlib is really old and out of date, no real reason to use it except legacy. I think most optimization effort goes into newer compression libs like zstd.
Legacy is a bit much. Deflate is probably the only supported compression format in many existing/live standards. For a greenfield project where you can control all clients, sure, use whatever, but zlib is likely going to continue to be used for decades.
I think you've confused Zstd (which they were not discussing) for Zlib (the primary implementation of DEFLATE, which is most commonly used by Gzip)
Gzip/DEFLATE is mediocre on most metrics relevant to compression but it's widely used of course. Zstd is better all-around, but much newer and thus doesn't have the same level of adoption.
The comparison is against zlib-ng, not the OG zlib. So this is still very impressive.
I suspect the results won't be quite as good on Aarch64 or other architectures though. zlib-ng has a pretty wide range of hand-optimized intrinsics whereas zlib-rs seems to only really have x86_64.