It sounds like you confirmed that it can't be self-hosted - are you sure about it not being true?
It's like saying git (observable) can't be self hosted because github (observablehq) is not open source.
But observable (the reactive notebook technology) can do everything a jupyter notebook can do, you just need to write a bit of communication code to hook up a websocket. (For me sandboxing the ui is a feature not a bug.)
And you can also self host it on a machine of your choice, and edit it locally.
We do that internally for all our dashboards.