I think the author is instead postulating that it is extremely dangerous because it's much easier to make an a-bomb with it than plutonium.
I suppose that's one way to look at it.
But there is a form of uranium that he may be thinking of. Not HEU but U-233. The critical mass is less than half that of HEU and it is easier to make because it doesn't require the enormously expensive enriching process.
It is produced by irradiating thorium in a nuclear reactor and fortunately that makes it radioactive to the point where it can't be handled.