For the libraries you listed, the benefits of using a native library are much larger, since they’re wrapping a well-known library that is known to be secure and fully-featured, or since the performance benefits are actually visible in any significant code snippet. But here, there is no Rust library to wrap, and I doubt the performance of a date-time library would have any effect on the performance of virtually all applications (maybe except for calendar apps).
datetime handling can absolutely be a hot spot, especially if you're parsing or formatting them. Even for relatively simple things like "parse a huge csv file with dates into dataclasses".
In particular, default implementation of datetime in cpython is a C module (with a fallback to pure python one) https://github.com/python/cpython/blob/main/Modules/_datetim...
Not saying it's necessarily justified in case of this library, but if they want to compete with stdlib datetime in terms of performance, some parts will need to be compiled.