In today's environment, an incredibly stupid thing to say. I have long heard the phrase "going off the reservation" as a metaphor, I am guessing he meant it that way, but there is no way you do that today. Things change, and I would doubt I would hear "going off the reservation" from anybody that had any sense about them now.
I am glad I am not a public figure. It must be exhausting to constantly catalog what you say. I try, but if I say something stupid, I would guess less than 5 people hear it, and fortunately, most of my friends/colleagues actually have forgiveness in their repertoire. The world at large has seemingly dispensed with that antiquated notion.
Context matters, words matter, position matters.
Terms like "going off the reservation," and "stay on the plantation" may have been acceptable and even common when he was younger but as we become more aware of the origin and impact of our words they aren't any more.
If they get casually used in certain settings they can be chalked up to simply not thinking but you hit it when you addressed being a public figure. He is likely the highest paid employee of the university, he is the face of the single most publicly visible element of the university.
Truth is that he isn't paid to coach basketball. There are plenty of highly qualified people out there who would gladly do the coaching part for much less money and could probably do that part of the job as well as he does.
His success on the basketball court is what keeps him employed but what he is paid huge amounts of money for is being that media presence, representing the university to boosters, alumni, potential students (and more than just basketball players,) being a leader of the group of young men who get the schools name in the national media frequently and on national TV more prominently.
He is paid to not say stupid stuff. He is paid to not put the school in a negative light. He is paid to not make this kind of mistake.
It may not seem fair but in our current environment saying that same thing would get many university presidents fired, same with many CEO's, broadcasters, and others who are paid to be public figures.
In the context of our times I wouldn't be surprised nor would I oppose his players choosing to not play for him.