Does "their stuff" refer to the atomic bomb they made ?

Context:

Warfare and weapons[http://en.wikipedia.org/w/index.php?title=Freeman_Dyson&action=edit§ion=19&editintro=Template:BLP_editintro]

On hearing the news of the bombing of http://en.wikipedia.org/wiki/Hiroshima:

I agreed emphatically with http://en.wikipedia.org/wiki/Henry_Stimson. Once we had got ourselves into the business of bombing cities, we might as well do the job competently and get it over with. I felt better that morning than I had felt for years… Those fellows who had built the atomic bombs obviously knew their stuff… Later, much later, I would remember [the downside].^[47]

I am convinced that to avoid nuclear war it is not sufficient to be afraid of it. It is necessary to be afraid, but it is equally necessary to understand. And the first step in understanding is to recognize that the problem of nuclear war is basically not technical but human and historical. If we are to avoid destruction we must first of all understand the human and historical context out of which destruction arises.^[48]