According to IPEP 17 (link), multiple worksheets on a single notebook will be deprecated, and a UI to handle them never existed either. Before, as in your link, the format of the notebook was
notebook["worksheets"]["cells"]
but it will soon be simply
notebook["cells"]
IPEP 17 is however an active IPEP, so changes may still occur at this stage (it is not accepted or implemented yet, see this list of IPEPs and their status)
EDIT: as a personal comment, having multiple worksheets like in Microsoft Excel may look like a nice idea, but you would often end up having many race conditions between sheets, with one cell of a worksheet depending on the output of a cell in another worksheet, often not in a self-evident way. And a scenario where many worksheets do not depend on each other is as good as a directory with as many notebooks.