Clustering is always a bit tricky when it comes to storage and networking.
Basically on the initial server, you weren’t prompted about anything because your only option at the time was dir and for some reason we don’t ask if you want to use an alternative source path when creating a dir storage pool. incus admin init doesn’t ask about every possible option as that would make it far too complex, so looks like it’s one of those where we decided not to prompt.
Incus then internally created your local storage pool which does mean assigning it a source path automatically (/var/lib/incus/storage-pools/local most likely).
When joining a new server, the new server must have the same network and storage setup as the existing server(s), so that’s why you’re not prompted about what you want to see created, you just don’t have a choice.
However Incus can’t tell if that existing local storage pool was created through incus admin init or later on by a incus storage create and therefore can’t tell if the source property is something that the user may want to customize, it therefore asks you anyway.
In your case, the correct response would have been to leave it empty, so just hitting enter when prompted for it.