The Star Wars universe is fictional. Do you honestly believe that the author from our universe who came up with this name was not aware of the (Earth universe) meaning of the word 'jizz'?
We can suppose that if the slang for sperm didn't exist that the writer would definitely have chosen "jezz", "juzz", or "jozz" instead, but personally I don't have any evidence to make that claim.
“Jizz” in English is etymologically closely related to “jazz” in English, which some sources also attest to having a (historical) slang use identical to that of “jizz”, so, while you are absolutely correct about the point of choosing the word, that point is quite compatible with the person doing so of being aware of the slang use of “jizz”.