It's not clear to me that the _entire_ Neon stack is OSS and available to self-host (though they do share a lot of OSS code, which is great), and in any case, it's not currently supported/documented beyond some "local development" instructions, e.g. "We do not officially support use of autoscaling externally" [0]
> Can’t you use a cloud provider and have them host this for you?
If it really is all OSS, then I guess the moat is the impressive execution of this team.
The only component which is not currently open source is the control plane.
There are some sections of the code that is close-source. Like the Console, but the devs are very active in their Discord. There is a small community trying to self-host, and from what I have seen gotten most of it working. There isn't an easy way to start it up for a more junior developer, but if you want to self-host it you are able to since all "components" are available