Thought it was totally clear.

It looks like and acts like it was built for Sonoma, not for Sierra.

As an IPython/Jupyter user for over a decade and Mac user for two decades, it’s not something I personally care about or an issue I’ve felt.

And the Jupyter notebook interface certainly isn't aesthetically pleasing. Colab is not as bad, but it's still not nice.

But some people do care about aesthetics, and this interface is actually aesthetically pleasing.