Your source is ignoring the fact that limitations on nuclear power in space are largely political. If we could put something bigger up there without concern it would be interpreted as a nuke, we would.
Cooling is hard in space. It is hard and expensive to get good power to weight ratios.
There's a lot of hard problems in space, though. We've solved a few of them - enough, at least, so that talking about large nuclear reactors in space should not be dismissed as a hoax.