Not exactly. There are two distributions of JupyterHub, but plenty of deployments don’t use a distribution at all. Especially those on HPC clusters with batchspawner, or custom deployments. JupyterHub is at its core is a regular Python package you can pip install, after all.
I don’t know if z2jh + tljh constitute a majority of JupyterHub deployments, but they definitely are far from all JupyterHub deployments.