This argument may be new to DeSantis, but it antedates the Civil War in the South.

Harriet Beecher Stowe included "good" masters in Uncle Tom's Cabin to refute the pervasive belief that slaves were better off under the protection of kind masters than they were free because they got medical care, were taught skills, etc.: Her theme was that slavery is wrong in all its forms (and even if, for example, slaves to develop skills).

This idea was enshrined after the Civil War in the "Lost Cause" myth and such things as monuments to Robert E. Lee. Many people in the Deep South still believe it, and DeSantis is pandering to them and to their descendants.