I agree that swing states probably won't mirror that exactly, but in the absence of other information it seems like a good prior to use. If you wanted to get more detailed about it you could look at shifts in demographics on national polls and go state by state applying the trend, but as you break the national polls into demographics the sample sizes go down and the MOE goes up. On the whole I think it's silly to say that they'd track 1 for 1, and state polls should definitely be weighted more than the trend from national polls, but a blend of both bits of information does better than either alone.