He's not rolling out his own libcrypt. He's fixing a bug: i.e. getting rid of the unnecessary dependency on a huge library (OpenSSH) just because of a hashing function. This is systems programming, not a brainless enterprise data pumping: you get rid of unnecessary dependencies if you want to run on cell phones and blade servers at the same time.
If all he wanted was to dump a dependency, it would have been easier to cut-and-paste a slightly slower implementation from another library. I think he's saying he enjoyed it. The benchmarks are just after-the-fact justification.
This provides a valuable insight into Linux culture. There have been many cases where the micro-optimization obsession has displaced effort from the "big picture".
A lot of times, micro-optimizations are more fun.
In a hobbyist culture, we tend to do what is most fun.