It is core. Mr Spivey accuses Florida of racism because Florida doesn't say it teaches something, but Mr Spivey doesn't prove that it's normal to teach it at all! I can easily imagine that literally no other states teach about slave rape because it's such an explicit subject. In order to prove that not doing something is unusual like Mr Spivey is accusing Florida of, you have a burden to prove that doing it is actually usual, but Mr Spivey doesn't do that.