“...There’s a much simpler way to explain how the golden shower incident wound up in the dossier. Emin Agalarov or his father Aras gave the story about the prostitutes to the same emissary they were sending to Trump Tower—Natalia Veselnitskaya. She gave it directly to Simpson. Someone embellished it along the line.”
There’s an even simpler way. It ever happened, and they simply made it up right there in their office. There was no “source”. A “source” is intended to make it seem possible on some level.
Or they were trolled by 4chan as has been widely reported - a joke that happened to fit perfectly into their preconceptions.