May the God he believes poisons everything be with him.

Hitchens is an atheist. Wishing God be with him during his illness is like wishing COBOL on a caterpillar or bequeathing morals to Newt Gingrich. None of them have any use for what you are wishing on them.
Sullivan knows that, of course. This is just the typical sort of supercilious passive-aggression that Christians specialize in, bless their hearts. Snide condescension disguised as caring. Wounded pride posing as generous pity. The proud thrill of fake humility. Rail all you want against God, puny human, but he still loves you and always will, despite your adolescent attempts to rebel against him. One day, perhaps, you’ll grow up enough to see this. Sullivan doesn’t care if Hitchens has any use for his useless well-wishing; what’s important is that he’s seen as offering it in the first place.
Besides, I believe the line Hitchens kept using in his book was “Religion poisons everything.” God, being nonexistent, doesn’t actually do anything. Religion, being the institutional structure built around humankind’s belief in an imaginary being, is what causes all the trouble.