Forgiveness comes with age. I have seen the similar trend of freed prisoners, who were exonerated after many years; having no anger or ill will toward the prison, guards, and judicial system.
True. Also, these interviews were conducted in 1937-38, 72+ years after the Civil War ended. Any former slaves who were still alive then would likely have been very young at the time they were freed (young enough that many may not have even understood their situation at the time).
Consider also that people who were there — including the slaves — had a better understanding of the underlying challenges the slave owners were dealing with. It’s pretty clear a lot of slave owners would have been happy to free their slaves by the time of the Civil War, because so many states had been passing laws that made freeing slaves more and more difficult!
Which goes back to the beginning of the country, really. George Washington couldn’t free most of his slaves because of the way the laws were written. And it was not entirely a myth that slave owners felt an obligation to care for their slaves, and some slave owners either hated slavery or tried to follow Biblical slave laws (meaning slaves could own property and earn money and were considered equal human beings in a temporarily subordinate role, etc.). No slave ever liked slavery, but the relationship could make the legalities of the situation less important.