Heh, that could certainly be possible. Though, given that the Bothan line was in Return of the Jedi and was about the plans for the second death star, it could also be excused as the Emperor revealed that the information leaked out intentionally in order to provoke the rebels into committing their entire fleet to an all-out assault.
It still seems like poor strategy on the rebellion's part to employ the services of such well-known spies in any event.
Given that the whole "all Bothans are spies" thing is an EU extrapolation and not backed up by any on-screen evidence, Mon Mothma's comment could also be construed to imply that the Bothans were not normally spies, but a collaborator race firmly on the side of the Empire. In that case, the few Bothans loyal to the rebellion would be in a prime position to pass on sensitive information.