I don't think that this cover is good evidence for anything. Sweet's art is unreliable, though he was asked to fix several inconsistencies for this cover, so that does help a small bit.

But more importantly, Mat isn't wearing his medallion in the Crossroads of Twilight cover (as far as I can see). Since he usually wears it under his shirt, it doesn't really seem far-fetched that cover art would leave out his medallion.