Doesn't that depend on whether or not they end up making a browser in the end?

Where is the end? And why claim you're doing the impossible until it's done?

It all comes off as puffery to me. Though the vertical slices approach is interesting, so I salute their efforts.

Well, it takes time to make something big. And, one way to do it is to choose end-to-end functionality. Idk, it doesn’t seem so controversial to me. I’d wait and see before calling it half-baked.

A vertical slice of the properly integrated features needed by some practical use case is certainly more efficient and effective than implementing the whole of a large API (e.g. some fancy recent, unproven and unstable CSS module) "in a vacuum", getting numerous rare cases wrong, and struggling to test the new features.