I can't find much information at all about these notebooks.
From what I can tell, they are Javascript only, which could make for an easier deployment story than a full Jupyter notebook server. That said, it's unclear from a quick tour of the site whether deploying these notebooks themselves is something one would or could do.
I find it odd that Bostock doesn't even mention Jupyter notebooks in the introduction. It is by far the most common and popular interactive notebook and supports many languages. There is also a next version - Jupyter Lab[1] which looks fantastic!
I do like the design of the notebook and the ability to pin cells. I got tired of Jupyter's horrendous default interface and wrote a new interface skin called Spin Zero[2]. I am trying to convince the Jupyter community to pay attention to their design.
And source: https://github.com/observablehq/notebook-stdlib