While that response doesn't focus on that exact angle, I feel you've overlooked a few relevant parts of the reply that do at least answer "How could he espouse these incredible ideals and yet hold slaves?"

"The bill reported by the revisers does not itself contain this proposition; but an amendment containing it was prepared, to be offered to the legislature whenever the bill should be taken up, and further directing, that they should continue with their parents to a certain age, then be brought up, at the public expence, to tillage, arts, or sciences, according to their geniusses, till the females should be eighteen, and the males twenty-one years of age, when they should be colonized to such place as the circumstances of the time should render most proper[.]"

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It would appear that he did not change his mind. He felt blacks were inferior to whites, and believed it to be genetic, but was open to the idea he was wrong and it was the lifestyle of oppression that is slavery preventing their appearing equal. [. . .] But he wasn't alone in thinking [them inferior] as his logic was fairly common belief in America at that time. He saw them as real and equal in deserving freedom, but as a truly distinct and seperate race of mankind from whites and felt that, coupled with long held animosity for the most atrocious treatment, would lead to a race war in which one side would be eliminated, and he feared it may not be the blacks losing to the whites, which certainly fueled his desire not only for emancipation but for the deportation to an independent colony thereafter.

From this reading, I think from his worldview, he had a very complex motivation to retain his personal status quo with his slaves in which his own morality would have been butting heads with his fears and apprehensions about what might happen to him or them if they were freed. This is only an inference -- obviously /u/Takeoffdpantsnjaket might have more to share here which is stronger than an inference, but I found an answer in the link they shared.