? I have not heard that indigo is soluble in urine. Urea is used for indigo to oxidize properly, and once it oxidizes and the garment is dyed, it wouldnt change anything, it's just another liquid, serving as a medium for the removal of dye.
Urine contains urea, which is why it was used in the oxidation process. You don't put the indigo in a vat of urine, you add some urea (urine) to a vat of water, with indigo. As I said, after it's oxidized, it's no longer especially soluble.