Nope, misunderstanding.
On Edge.Cuts I just draw simple line and arc segments.
The Polygons are only on the copper layers as usual.
The link below is an example where I drew a pentagon zone outline around the board, and let Pcbnew clip the polygon to within the board edges.
I have never seen KiCad go wrong on this, so I can not give an example of this. Another reason to do this is for example if you have multi layer boards. If you look at the Olinuxino A64 (On github, also link from KiCad website under “made with KiCad” they also use this. They have multiple power zones on different layers, and putting the edges way beyond the outline of the PCB makes each zone easy to select for editing it’s parameters.
Again: There are no polygons on Edge.Cuts, just simple lines at the moment. So your options are to simply use those simple lines and arcs, or use an external CAD program and import outlines via dxf format.
There is also an option in Gerbview to back-import Gerbers back into a KiCad project, which may be handy for reverse -engineering or re-using board outlines, mounting hole locations etc from other PCB’s even if made in a completely different PCB program.