This appears contradictory and to confuse function with purpose. The patch
does position the bullet (as you concede in your second statement) -- and positions it centrally in the barrel diameter (to at least some degree of precision). So it
does "ensure correct positioning". That's a
function of it, and that definition of "sabot" is a functional definition. The fact that the primary
purpose of the patch may be to act as a gas seal in no way detracts from its function as a sabot. A sabot in a rifled barrel also serves as a gas seal -- which may be argued as its primary
purpose since it is primarily employed to allow the firing of sub-caliber projectiles in a barrel. Indeed, a patch and a sabot appear to have the same primary purpose: to serve as a gas seal in order to fire a projectile that is smaller diameter than the lands of the barrel. So I'm coming down on the "patch is a sabot" side of this one.
Also, some definitions are better than others, and that definition isn't particularly good. Neither is the one in Wikipedia. They're not really definitions, but descriptions -- and emphasize specific aspects of the term.