Technically this geometry is planar because you can fit a plane through all the points, the points just don't define a single plane as it's effectively a line.
As for the analyze tool in V8i, I didn't step into it to see what it's actually doing, but since "Non co-planar shapes" is being reported as the "Area" property, it's probably just going off the fact that it's a "closed" element type and mdlMeasure_areaProperties either failed or returned 0.0 for the area.
I don't agree with what the analyze dialog is showing in V8i, because I do consider this planar geometry. It should just report an area of 0.0 (which is what the Connect edition does).
If you did want to explicitly test for this case, you would need to test for colinear points using something like mdlVec_colinear or mdlElmdscr_evaluateCurveSpace (your understanding of what the 1 return value means is correct).
HTH
-B