The IJulia kernel just sits there listening on ZMQ — it doesn’t use Python or Jupyter directly, it just talks in the Jupyter protocol. Something other than a local Python-based Jupyter front-end might communicate with the IJulia kernel: a remote Jupyter client, Hydrogen, nteract, …
Previously, IJulia required Jupyter to be installed in order to install the kernel specification via jupyter kernelspec install. However, since the installation location is documented and standardized, and “installation” consists of just writing a JSON file to the right place, it was straightforward to do this manually and remove the jupyter dependency.
There is still an IJulia.notebook() function that launches the notebook, which requires jupyter notebook, but this now prompts you for whether to install Jupyter (via Conda.jl) at runtime when notebook() is executed, rather than in Pkg.build("IJulia"). People who don’t want IJulia to install Jupyter can just run jupyter notebook themselves in whatever way they like.