No, it's a nearly complete open-source reimplementation of the full operating system and kernel. I don't know where you're getting the idea that it's a patch set - you can take a look at the roadmap and design docs here (or just browse the source) if you'd like.
The readme literally lists a couple driver changes to make CFWs easier, breaks in the boot and crypto chain of trust, and "Application-level Horizon OS patches, used to implement desirable CFW features".
That's not a "nearly complete reimplementation".
A bunch of not started work in a planned features doc doesn't change that.
https://github.com/Atmosphere-NX/Atmosphere