The point here is that they've unified the codebases. The application "Jupyter Notebook" is just a single-document version of "JupyterLab", designed to just do that one part of Lab.
Previously there was "Jupyter Notebook". Then they separately wrote JupyterLab (creating a brand new implementation of notebooks for it). Now, they're taken the JupyterLab notebook code and used it to replace "Jupyter Notebook".
Thanks for explanation. I still don't get what is the motivation to keep Jupyter Notebook going. Is it different feature wise? Or it's just a chrome over JupyterLab because people like the retro look of Jupyter Notebook?
Previously there was "Jupyter Notebook". Then they separately wrote JupyterLab (creating a brand new implementation of notebooks for it). Now, they're taken the JupyterLab notebook code and used it to replace "Jupyter Notebook".