I cannot prove his source did not exist.
The question in my opinion is not really relevant. Beside given the manuscript evidence and all the other evidence as well, the better question would be is there any reason to believe it did existed as part of the text?
He may have quoted an old latin manuscript.
Not impossible but the majority of scholars believe it was a gloss, a marginal note, that was copied into the body of the Latin text.
However I believe he was interpreting the Verse.
The evidence points to that very explanation.
Note how Cyprian phrases it: again it is written
of the Father, and
of the Son, and
of the Holy Spirit, "and these three are one."
Also note the word
de in bold.
Et iterum
de Pater et Filio et spiritu sancto scriptum est: Et hi tres unum sunt.
The
de is a Latin preposition meaning "about." So the Latin reads: "it is written
about the Father, and Son, and Holy Spirit." I think it is clear that this section is not a quote but an introduction to the quote "these three are one." Thus it is better to understand it as Cyprian’s interpretation of 1 John 5:7-8.