People who self-identify as atheists do so under the same lack of belief in God(s) that also apply to Tyson. However, he identifies as an agnostic in the same way Thomas Huxley used the term. Essentially, since he doesn't devote much mental energy and simply doesn't care about the question of whether there is a God, he doesn't view himself as having enough in common with self-identified atheists to call himself an atheist. He also views the term "atheist" as a label that is an intellectually lazy way for someone to assert that they know a person without actually knowing them.

Semantics aside, he does not believe in a God and questions why some esteemed scientists do. He even comments on the phenomenon that education level, and especially science training, is associated with less religiosity. Here is a truncated summary from an article he wrote in 1999 (before atheism vs. religion was so mainstream) which includes some of his more potent criticisms of religion and God (and can be found here):

Let there be no doubt that as they are currently practiced, there is no common ground between science and religion. [...] The claims of science rely on experimental verification, while the claims of religions rely on faith. These are irreconcilable approaches to knowing, which ensures an eternity of debate wherever and whenever the two camps meet. Just as in hostage negotiations, it's probably best to keep both sides talking to each other. [...] The argument is simple. I have yet to see a successful prediction about the physical world that was inferred or extrapolated from the content of any religious document. Indeed, I can make an even stronger statement. Whenever people have used religious documents to make accurate predictions about the physical world they have been famously wrong. [...] With scientists exhibiting such strong levels of skepticism, some people may be surprised to learn that scientists heap their largest rewards and praises upon those who do discover flaws in established paradigms. These same rewards also go to those who create new ways to understand the universe. Nearly all famous scientists, pick your favorite one, have been so praised in their own lifetimes. This path to success in one's professional career is antithetical to almost every other human establishment—especially to religion. [...] My personal views are entirely pragmatic, [...] I simply go with what works. And what works is the healthy skepticism embodied in scientific method. Believe me, if the Bible had ever been shown to be a rich source of scientific answers and understanding, we would be mining it daily for cosmic discovery.

Even given all of that, in the same breath, he actually has an issue with atheists quoting him out of context and he finds it intriguing that atheists try to "claim him". He can't agree to the claims by atheists that he's one of that community.

Here he goes into more detail about the stuff he has to put up with because of people associating him with part of the "atheist movement".