Initially I thought this was a joke (Julia doesn't seem to be popular enough to be one of the cornerstones of Jupyter, compared to Python and R), but indeed, Jupyter's documentation says it's true:

> The name Jupyter comes from the three programming languages the project originally supported: Julia (ju), Python (pyt) and R (r).

https://docs.jupyter.org/en/latest/what_is_jupyter.html

I think it was a bit unclear, but from the mention of the benefits of "dogfooding", I think they're talking about how the underlying infrastructure of Jupyter is written in Python.

Yes, Julia works fine in Jupyter notebooks as a kernel, but the Jupyter notebooks itself is implemented in Python.