Atheists are often found arguing like this:
(1) If intelligent design is true, then there must be a designer.
(2) But ID is actually creationism in a cheap tuxedo. (self-evident)
(3) So then, the designer is the Christian God. (from (2))
(4) But a Christian God would not design a world with so much evil. (self-evident)
(5) Hence the designer does not exist. (from (4))
(6) Hence no design. (by contraposition, from (1))
This argument can, of course, be challenged at (4). But the main problem is (2) and its corollary, (3). A supporter of ID may in the privacy of his heart believe that the designer of biological systems or physical laws is, in fact, the Christian God. But ID as such does not support this conclusion. If we use it to do natural theology and at the same time adopt some form of scientific realism, then we might be able to conclude that the designer is an immensely intelligent and powerful being; but it is easily possible that we will not be able to advance much beyond that. In other words, ID as a science is compatible with a designer (or designers) with many different sets of attributes. It does not give us the Christian God but only a fairly thin slice of something intriguing and curious beyond.
The tu quoque mentioned in the title says that if a theist cannot use God in science, then neither can an atheist. In other words, an atheist cannot appeal to the problem of evil or to suboptimality of design in arguing against the possibility of design in nature, the methodologies of detecting design, or against any particular design inference.