If there is a large volume of fluid, there is only one place that can originate. It may include other stuff, but there is no other reservoir of fluid in the neighborhood...
Of course, women do secrete various fluids during sex, lubricant and ejaculate, but they are in very small volumes because the organs producing them do not produce in quantity and do not have large reservoirs in which to store the fluid.
FWIW, I have been squirted on and it did not seem like urine to me (lighter and seemed less salty), but then urine will vary greatly depending on what and how much you have been drinking so without interrogating the lady in question on recent fluid intake (I chose not to do so), it seems safe to assume she had drunk a lot of clear fluids before we met in order to execute this performance.
Not that it makes any difference as I did not find it enjoyable enough to ever want to repeat.
One could conclude other things from the Wikipedia article since it seems to be a list of contradictory studies and opinions, but I am going to stick with the source issue - where could the fluid come from, and how did it get to the vagina to be ejected. Not a lot of options there.
Yes, I know, discussed many times before. You either believe or you do not, and that probably depends to a great degree on whether you like the experience or you do not. I mostly disbelieve, and did not like the experience.
zig