Great that someone is thinking about this, doesn't seem very usefull though, it requires MacOS. But the bash one has been there since the beginning, so you are comparing a project that seems to be created in 2021 without any need for backward compatibilty with one that has grown organically for 15 years.

It doesn't have to be pure shell. I've written a few tools that have built-in completion helpers. All their completion scripts have to do is run the tool with the right environment variables, and return the results.

That's also how autocompletion works with Python's excellent Click library.