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".
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".