The thing is they still were American scientists, not German imports as were our rocket scientists after the war. Don’t nit pick, it is the sign of a loser.
Sorry, but those who picked nits with you are in the right.
A very large percentage of the most critical scientists in the Manhattan Project were refugees from the Nazis. They were from a variety of European countries. Many were Jews and/or (unfortunately, as it turned out) socialists or Communists.
They were especially dominant in the theoretical physics side. Prior to the war America just didn't have much in the way of theoretical physicists, with those we did have largely imported.
Most of these scientists became naturalized citizens eventually, but it is really inaccurate to say we used "all American scientists" to build The Bomb.
BTW, British and Canadian scientists played a significant role too.
Most of them were not naturalized until the early 40s, and two of the most important, Enrico Fermi and Leo Szilard, were not naturalized until well after the Manhattan Project was underway. They were no more or less American than Mr. Mascheroni.