Not credible at all.
Detonating one nuke in a US city is sufficient end in itself. The impact would be profound. The efforts to prevent it are enormous. To have one in place ready to blow, but sit on it and wait for placement of others risks losing any opportunity. Sending demented handwitten screeds with deranged claims is worthy of the dime-a-dozen nutcase pile. A letter means nothing. A mushroom cloud over a city makes and changes history.
I think the number is credible. They are not referring to traditional nuclear warheads. They are referring to dirty bombs, the way I read it.