Download code doesn’t really download a webpage. Rather, it downloads your notebook together with the Observable runtime and other files required to run the code locally. You can also use the downloaded version to embed any components of the notebook that you want into a webpage, with depending on observablehq.com.
Hmm if I may submit feedback here, it should be possible to download only the chart too. I think that Observable’s features are awesome but if I’m just trying to get a chart working in my local environment this (as only option) is very inconvenient.