Yup, I know, it doesnt check out at all, you wouldn’t put a rotary on a helicoptor. But “rotary” engines were indeed used on aircraft, since they also refer to a radial cylinder configuration where the engine housing moves and the axis is fixed. So I think it’s likely they were referring to the common WWI plane engine. I didnt think of a wankel at all till you mentioned it! Interestingly enough, modern wankel engines are also sold for helicopters.