If theistic belief is false, then it is irrational.
Theistic belief is false.
Therefore, theistic belief is irrational. I think that's right. Perhaps they are simply arguing both, that theism is false and that it is irrational. And it may be that some of them take it that what they have provided is an undercutting defeater via the argument from evil, rather than a rebutting defeater. Remember, quite a few atheists take their position not to be a denial of God's existence, but a withholding of both theism and its negation.