Viruses behave very differently in different species. For example, I think ebola can be airborne in pigs because it binds to receptors in pig lung cells and is more of a respiratory disease for them. It doesn't have a great affinity to human lung cells however so it's NOT airborne in humans.
I'm not suggesting that either fluid was the pathway by which the virus jumped to humans, nor do I know how it happened, only that evidence suggests researchers interact closely with the animals.
And I am not ready to dismiss the theory but I am always open to hearing evidence to exclude the theory.