As phamt wrote, I had no reason to assume that my testcases were wrong, because I passed the judge while testing against these cases myself.
So, maybe Wikipedia is wrong in this case (very possible), but it says a polygon is a "plane figure that is bounded by a finite chain of straight line segments closing in a loop" (https://en.wikipedia.org/wiki/Polygon). To me that sounds like like a line (forth and back) is also a polygon (with area 0).
Anyway, I had no intention of confusing anyone. You could also just have asked for clarifaction and we could have discussed the desired "correct" outcome.