Remember that Marziano explicitly states that he is going to ignore the multiple numbers, and conflate them. Thus he seems to conflate the Jupiters, for instance the "King of Athens" and specific laws in GDG II, 2 (although second Jupiter is also called a king of Athens V, 1) with the third or Cretan Jupiter GDG XI, 1, who is given as husband of Juno, the War with the Giants, etc.
For Venus he seems to conflate even more, the heavenly Venus "Lucifer" with the mother of Cupid, the third Venus. Here is the short passage in Cicero that Boccaccio relies on.
Cicero, De natura deorum
, III, xxiii (59); page 342-345 of Harris Rackham edition and translation, Loeb, 1933.
https://archive.org/details/denaturadeo ... 4/mode/2up
The first Venus is the daughter of the Sky and the Day; I have seen her temple at Elis. The second was engendered from the sea-foam, and as we are told became the mother by Mercury of the second Cupid. The third is the daughter of Jupiter and Dione, who wedded Vulcan, but who is said to have been the mother of Anteros by Mars.... The first Cupid is said to be the son of Mercury and the first Diana, the second of Mercury and the second Venus, and the third, who is the same as Anteros, of Mars and the third Venus.
Venus 1, GDG III, 22
Venus 2, GGD III, 23
Venus 3, GDG XI, 4
Cupid 1, GDG II, 13
Cupid 2, GDG III, 24
Cupid 3, GDG IX, 4
Latin, opening paragraph -
De Venere Iovis XIa filia, que peperit Amorem.
Venus, Omero teste, Iovis fuit filia et Dyonis, et est hec, quam Tullius, ubi De naturis deorum, terciam vocat. Et coniugem dicit fuisse Vulcani. Hanc aiunt Martem amasse, de quorum adulterio et captivitate supra ubi de Marte dictum est. Sic et Enee matrem dicunt, de qua etiam ubi de Enea scriptum est.
The opening paragraph of the Third Venus in the 1498 French translation of GDG -
https://gallica.bnf.fr/ark:/12148/bpt6k ... geneologie
"Venus, eleventh daughter of Jupiter who gave birth to Love.
De Venus vnziesme fille de iupiter laquelle engendra amour. Chapitre iiiie.
Ainsi que homere tesmongne Venus fut fille de iupiter & de dion Et est celle, laquelle tulle en son liure de la nature des Dieux dit estre la tierce & auoir este femme de vulcan. Ils dient quelle ayma mars, de ladultere et captiuite deulx est la hault escrit la ou est parle de mars. Pareillement dient de la mere de eneas, de laquelle est aussi escript la ou est fait mencion de eneas.
"As Homer attests, Venus was the daughter of Jupiter and Dion, and it is she whom Cicero in his book On the Nature of the Gods calls the third, and says to have been the wife of Vulcan. They say that she loved Mars, concerning whose adultery and captivity it is written above where it is spoken of Mars. Also they call her the mother of Aeneas, of whom it is also written where Aeneas is mentioned."
Obviously Cicero knew nothing of Venus as mother of Aeneas, but Marziano knows the Venus who is the mother of Cupid by Mars, which is Boccaccio's and Cicero's Third Venus, who is also the mother of Aeneas. It would seem that Castelletto (and Galvano Fiamma who created the basis that Castelletto filled in a little) could only have been thinking of the third Venus and the Cretan or third Jupiter.