It raised a flicker of polite interest that died, from your own link:

    The Advisory Committee on Uranium was the beginning of the US government's effort to develop an atomic bomb, but it did not vigorously pursue the development of a weapon.

What did move the needle, again from your link:

    The Frisch–Peierls memorandum and the British Maud Reports eventually prompted Roosevelt to authorize a full-scale development effort in January 1942.

In essence, the letter from Einstein did not start the Manhatten Project, it was instead the direct efforts and in person meetings of Oliphant:

https://en.wikipedia.org/wiki/Mark_Oliphant#Manhattan_Projec...