What do you mean by people with bipolar disorder being sane?

I could understand the claim that depressed people have accurate beliefs, but are a danger to themselves, but you said bipolar. Manic stages definitely involve false beliefs and overconfidence. Maybe they'd be considered within the sane range if the overconfidence were merely verbalized. Acting on beliefs that lots of people verbalize is often insane.

Or maybe I my parse failed...an individual with bipolar is sometimes sane and sometimes insane. But I'd still call the individual insane.