Who Knows, I checked the quotes in
The God Delusion and they are selective. Hitchens has not established that Jefferson was atheist either. On page 42 of TGD Dawkins writes that Hitchens "thinks it likely" that Jefferson was atheist. Then on p.42 Dawkins draws no definite conclusion, writing "whether Jefferson and his colleagues were theists, deists, agnostics or atheists....". (He repeats this on p.45) In other words, he does not know. Hitchens himself wrote, "as to whether he was an atheist, we must reserve judgement...". I don't think Dawkins was jumping to any conclusion.
In a letter to Charles Thomson 9 January 1816, Jefferson wrote:
[The Jefferson Bible] is a document in proof that I am a real Christian, that is to say, a disciple of the doctrines of Jesus, very different from the Platonists, who call me infidel and themselves Christians and preachers of the gospel, while they draw all their characteristic dogmas from what its author never said nor saw.
As I said, his views were complex. In some writings he condemns Christianity, and in others he praises it. The subtle definitions were his own making, he saw both good and evil in Christianity. For example:
Millions of innocent men, women and children, since the introduction of Christianity, have been burnt, tortured, fined and imprisoned; yet we have not advanced one inch towards uniformity.
-Thomas Jefferson, Notes on Virginia, 1782
And again:
In every country and in every age, the priest has been hostile to liberty. He is always in alliance with the despot, abetting his abuses in return for protection to his own.
-Thomas Jefferson, letter to Horatio G. Spafford, March 17, 1814
It is clear that Jefferson was antagonistic to organised religion which became corrupted by power and greed. But he
did believe that Jesus was the "incomparably great teacher", whose teachings were not being followed by the priests.
He also had this to say about atheism:
If we did a good act merely from love of God and a belief that it is pleasing to Him, whence arises the morality of the Atheist? ...Their virtue, then, must have had some other foundation than the love of God.
-Thomas Jefferson, Letter to Thomas Law, June 13, 1814
Whatever you read, read with a critical eye, and preferably from primary sources. Bias is everywhere.
Edit to add another Jefferson quote:
I can never join Calvin in addressing his god. He was indeed an Atheist, which I can never be; or rather his religion was Daemonism. If ever man worshipped a false god, he did.
-Thomas Jefferson, letter to John Adams, April 11, 1823