I don't see a problem with a satanist being an anti-theist.  If somebody believes in the biblical god, he presumably believes that Satan is not a god.  However, since a Satanist also presumably believes that Satan is in the right and God is in the wrong, he is against the worship of God -- theism.  I think the problem you're seeing with Hitchens statement is that there's a slight equivocation on the term.  In my book, belief in the existence of a deity makes you theist.  You don't have to practice religion or even like the deity you believe in.  You just have to believe it exists.  By this definition, a Satanist is a theist.  However, if we narrow the usage a little bit, so that theism is the practice of trying to do what you believe a deity wants you to do... well, that's a little muddy.

Admittedly, this is a stretch.  I don't think most modern satanists (I think that accounts for what... about twenty five people now?)  actually believe in God and Satan.  The three or four satanists I've talked to in recent years seem to believe in a philosophy more than an actual being.  In any case, from a strictly hypothetical point of view, sure.  Satanists are anti-theist, and I don't see how to avoid calling them theists either.

We also have to realize that one could be an atheist without being an anti-theist.  There are clearly some people who believe that practicing religion is good for humanity even though they don't believe there are really any gods out there.  The myth that religion is good for morality has a long and glorious history, and there are certainly nonbelievers who buy into it.

I'm certainly an anti-theist, but I've always had a clear distinction between atheism and anti-theism.  Atheism is the result of properly applying the scientific method to the universe.  Anti-theism is more of an inexact position since theism manifests in so many ways.  As Cpt_Pineapple will quickly point out, a blanket declaration against any and all theism is not as easy to justify as atheism itself.  I try to keep those two conversations completely separate just to make sure that no cheeky theists will accuse me of being atheist because of my anti-theism.  Quite the contrary is true.  I was an atheist for several years before the accumulation of evidence convinced me that theism is inherently bad for humans.

No, I see a big a equivocation . I understand that it would be horrific to have the God of bible continually controlling and watching us. But the average fundamentalist is just going to read the title of his book 'God is not Great' and hear him say he thinks having God rule the universe would be horrific. Then they say, see "all atheists hate God", just like the bible says. And never understand that we can't hate something we don't believe exists.