Since you like the program, it would be nice to be considered! But, are you willing to pay for some of the expensive tuition, fees and stipend? To answer your question, I think what the political science department does is just to forward your application to the graduate program. I don't think you can actually consider this e-mail as an acceptance since another whole round is waiting for you.
By the way, when did you get this e-mail? I applied to Yale as well for Ph.D. in political science, but didn't get this e-mail. Perhaps, my application is not even recommended for other departments!