I assume the complainant believes Thackery was excusing slavery since the slaves he found here were physically and mentally healthy. But I interpret the quote to have a nearly opposite meaning.
First, “not repulsive” is hardly a ringing endorsement. Try this: “Will you take my cousin out on a date?” “What’s she like?” “Well, she’s not repulsive.”
More importantly, I infer he was surprised by the health of the slaves he met in Augusta. This means in all other places he had met slaves, they were not healthy and he was repulsed. He actually was condemning slavery with this “compliment” to Augusta.