They probably could have explained better what hasn’t changed. Observable Notebooks work like a spreadsheet or makefile. The cells aren’t executed top to bottom (like a Jupyter Notebook). Instead they’re executed in dependency order.

Looks like this is still true with the new system:

> The Build API analyzes cell source code to find unbound references, such that the appropriate variable graph can be initialized using the Observable Runtime. For non-JavaScript cells (such as Markdown, HTML, and SQL), the Build API also transpiles the source into a tagged template literal expression.

This is unlike other document formats. They made the cells look like standard JavaScript now (versus the JavaScript variant they had before) but the file format is necessarily different, because the execution model is different. There is a build step to ensure that the cells will be executed in the right order on page load.