I would not be surprised if this was true, or at least had a significant amount of truth to it. It reminds me of the Zimmermann telegram that brought the US into WW1. The UK and the US both knew that Mexico was not going to help Germany (what with a Mexican revolution going on and all that), but the telegram was a convenient enough excuse for the US to join the war in Europe anyway.
The parallel here is that the US and UK are both waiting for the enemy to make a huge blunder (Germany trying to get Mexico to attack the US, or Japan attacking Pearl Harbor). Getting the US into the war was one of the top priorities of Churchill's government. And Roosevelt really wanted to help (the Lend-Lease among other things).
So this information, that Japan is about to attack, comes along, why would you stop it? Plus; how would you stop it? You can argue that revealing it was about to happen publicly would be enough for President Roosevelt to argue a war against Japan. But maybe he felt if Japan actually got to attack, he could also argue for a war against Germany?