The lower back and loins go to Libra by the positional rule, and the kidneys come with them. But the kidneys are also given to Venus in the parallel scheme of planetary rulership of the organs, which this atlas records on the Organ's own page, and the two claims are made by the same authors in the same books without any sense of conflict.
That is not carelessness. Medieval and early modern astrological medicine ran two mappings at once and used them for different purposes. The zodiacal mapping is positional and answers the surgeon's question: which member is at risk today, given where the Moon is. The planetary mapping is temperamental and answers the physician's question: what is this organ's nature, and what humour does it govern. A part could be Libra's for the first purpose and Venus's for the second with no contradiction, and in the case of the kidneys the two even agree, since Libra is itself ruled by Venus.
Where they do not agree the atlas records both, because collapsing them would misrepresent how the material was used.
Neither mapping is a statement about renal function. This entry records a historical doctrine of correspondence, and nothing in it is medical guidance.