The best defenders are (as you also said) often high up in the plus minus for their teams. That logic does not reverse for bad teams, even if the starter vs starter stats turn into negative ones. See Dunn and Kessler last year, both of whom started plenty of games. Tech also was one of the top plus minus guys, but his full season numbers arent comparable.
I'm not comparing Ochai to their bench, but to Dick, Mitchell and Poeltl. Those guys have gotten the bulk of the starter vs starter minutes alongside Ochai, yet arent nearly as negative on the box. And Ochai's efficient stats on offense actually boost his +-, as inefficiency on offense is something that really hurts those numbers (see Keyonte).