One is that the number one Science and Engineering powerhouse prior to WWII was Germany, not Britain.
Two this totally neglects that the US received the lion's share of Scientists and Mathematicians from countries like Germany, Hungary, Poland etc with the encroachment of the Soviets and persecution of the Jewish people.
While the down up approach of the US and heavy funding probably helped a lot. Bringing in the Von Neumanns and Erdos of the world couldn't have hurt.