It may perhaps be worth remembering that Wirth designed the images about 30 years before his book came out (if I recall the dates correctly).
This is taking the cards as dating from the late 1880s, and his book from the mid 1920s.
Though of course Wirth continued to be guided by the works of Guaita, Christian, Papus and Levi, he also developed his views as expressed in his wonderful book.
At the time of the deck's design, I would suggest, contra
Ayumi's suggestion, that it was not so simple that "Most of the occult 'gurus' of the day were associating the Empress with Venus, and the Papess with the moon".
For example, though Papus does make that association, he also suggests that the Empress has at her feet the Moon.
Interestingly, by the way, the images Papus uses of the 'first' version of the Wirth deck has the 'horns' of the Moon pointing UPWARDS
, not down as in later versions (Cf pp 91 and 115 of The Tarot of the Bohemians
In any case, the clearest reference I have come across for the reference to the Apocalypse (though indicated in Levi), is from Paul Christian's History and Practice of Magic
, p 97:
"Arcanum III is represented by a woman seated at the centre of a blazing sun; she is crowned by twelve stars and her feet rest on the moon. [...] This woman, celestial Isis or Nature [...]. The moon beneath her feet signifies the weakness of matter and its domination by the Spirit"