This was after Hiroshima and Nagasaki, in a secret, government lab staffed by the utmost experts.

It wasn't the "oh look, something funny happens if I do this!" stage of experimentation. This was after they understood what they were dealing with well enough to build and successfully use two bombs. And Slotin was supposedly about to move elsewhere and was working on passing on knowledge.

That's why it's so weird to me.