It's only sort of true. One thing to note is that spontaneous fission occurs all the time with naturally occurring U-235, but at extremely low rates such that the buildup of fission byproducts happens only in trace quantities. But if it didn't occur then fission reactors would be far more difficult to build since there wouldn't be a small background of neutrons from fissile materials to be amplified by reactor/bomb designs.

As to natural reactors, those did exist and they did produce Cs-137, naturally. However, those reactors ran only billions of years ago, so the radioactive byproducts, such as Cs-137, have long since decayed.