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At the same time, I bristle slightly at his comment, ‘Jefferson was not, in my opinion, a genuine Christian.’ I do not believe that anyone is capable of evaluating anyone else’s commitment to his or her religious beliefs.

Except that Dr. Kennedy, according to his column, hadn't arrived at his conclusion based on Jefferson's behavior, but based on what Kennedy alleges Jefferson said. If someone rejects Jesus' divinity, I'd say that objectively makes him not a Christian.

So the question comes down to, Did Jefferson say what the columnist alleged he said?

Unitarians did (and do) reject the divinity of Christ. Jefferson was a Unitarian.From Channing's 1819 sermon Unitarian Christainity:

". . .we believe in the unity of Jesus Christ. We believe that Jesus is one mind, one soul, one being, as truly one as we are, and equally distinct from the one God. We complain of the doctrine of the Trinity, that, not satisfied with making God three beings, it makes; Jesus Christ two beings, and thus introduces infinite confusion into our conceptions of his character. This corruption of Christianity, alike repugnant to common sense and to the general strain of Scripture, is a remarkable proof of the power of a false philosophy in disfiguring the simple truth of Jesus.
According to this doctrine, Jesus Christ, instead of being one mind, one conscious intelligent principle, whom we can understand, consists of two souls, two minds; the one divine, the other human; the one weak, the other almighty; the one ignorant, the other omniscient. Now we maintain, that this is to make Christ two beings. To denominate him one person, one being, and yet to suppose him made up of two minds, infinitely different from each other, is to abuse and confound language, and to throw darkness over all our conceptions of intelligent natures. According to the common doctrine, each of these two minds in Christ has its own consciousness, its own will, its own perceptions. They have, in fact, no common properties.
The divine mind feels none of the wants and sorrows of the human, and the human is infinitely removed from the perfection and happiness of the divine.
Can you conceive of two beings in the universe more distinct? We have always thought that one person was constituted and distinguished by one consciousness. The doctrine, that one and the same person should have two consciousness, two wills, two souls, infinitely different from each other, this we think an enormous tax on human credulity. . .
We believe, then, that Christ is one mind, one being, and, I add, a being distinct from the one God. That Christ is not the one God, not the same being with the Father, is a necessary inference from our former head, in which we saw that the doctrine of three persons in God is a fiction.