The line between client and kernel is intentionally fairly austere: the linked code works today in e.g. JupyterLab and the IPython terminal app… but probably not nbconvert, for example. I would also probably not expect this to be extended in the future.
One approach to bridging the two is a client-specific tool such as ipylab: it can run any jupyterlab command… but there might not be one for “new markdown cell”.