It's implied by the fact that early in the post they say:

>"To test this system, we pointed it at an ambitious goal: building a web browser from scratch."

and then near the end, they say:

>"Hundreds of agents can work together on a single codebase for weeks, making real progress on ambitious projects."

This means they only make progress toward it, but do not "build a web browser from scratch".

If you're curious, the State of Utopia (will be available at https://stateofutopia.com ) did build a web browser from scratch, though it used several packages for the networking portion of it.

See my other comments and posts for links.