It may help you to also examine the classes of the Ruby API, as it more closely resembles the OOP C++ core of the SketchUp model.
With regard to the
Sketchup::ArcCurve) class objects, … they will expose the center point, start and end angles, radius, as well as other properties that basic curves do not have … (basic curves can be non-circular, … ie, bezier curves, freehand curves, sine curves, etc. etc.)
The SketchUp APIs do not actually have polygon or circle objects as low level class objects. Instead they are just a closed set of equal length edges that are grouped into an ArcCurve object. (Their start and end vertices will be the same 3D point. The difference between the start and end angles should be 360 degrees.)
The Ruby API has a boolean
Sketchup::Curve#is_polygon? method, but the C API does not have a complimentary function.
The C API has a
SUArcCurveGetIsFullCircle function, but the Ruby API does not have a complimentary method.
I see you’ve opened an API issue last February …