FFMPEG uses hand written assembly across their code base, and while 94x may not be representative everywhere, it's generally true that they regularly outperform the compiler with assembly:
^ this, took my biggest step entry into programming via learning how to get my videos on an iPod Video in 2004, that eventually required compiling ffmpeg and keeping up with it.
I'll bet money, sight unseen, that poster above is right its used for HEVC. I'll bet even more money its not some massive out of nowhere win, hand-writing assembly for popular codecs was de rigeur for ffmpeg. Thrust of the article, or at least the headline, is clickbait-y.
https://x.com/FFmpeg/status/1852913590258618852 https://x.com/FFmpeg/status/1850475265455251704