Herman Melville's novella "Benito Cereno" is considered an example of an unreliable narrator, in this case the American Captain Delano of the rescue ship. Since your question asks what Melville himself thinks about the personhood/humanity of the Africans on board the Saint Dominick and since we have an unreliable narrator, we will have to do close reading to try to find hints about Melville's attitude—but first we need to unpack the slavery/humanity/personhood aspects of your question.
Slavery is a legal status of a person, a status that has existed throughout recorded humanity and that still exists in some form in some cultures today. In other words, it is the laws of a country that allow or prohibit slavery and the enforcement of those laws that permit it or not. Melville brings this to our attention when he writes,
Poor Babo here, in his own land, was only a poor slave; a black man’s slave was Babo, who now is the white’s.
Legality is a separate philosophical question from morality, at least since the nineteenth century. But Melville, writing in 1855, is arguably an abolitionist simply by writing this work, since pro-slavery advocates would not want to draw attention to slave-trading by writing about it because there is no positive way to portray it. However, since we are trying to determine Melville's own perspective from this work, we need to understand that some abolitionists were still racists, while others were not.
A racist abolitionist of 1855 would want to abolish slavery for a number of reasons, yet they would feel that black people were somehow inferior to white people, so can we find instances in the novella where the Africans are described by Melville as inferior or less than human? If we do find some potential examples, we must then determine how much of Melville's negative inference is due to the revolt dynamic underway instead of a basic disregard for the humanity of the Africans. Keep in mind that Melville, the great maritime author, is writing about an event at sea where the captain is legally the sovereign.
This is an interesting question that will involve close reading and interpretation to answer.