I agree, "night and day". A great spey rod does not perform overhand necessarily well, nor the converse. Although an incidental few do both well, this dual-purpose is not a criteria I see people seeking out since they are looking to either spey cast or overhand cast with two-hands.
I think it's a misnomer that the folks who use two-handed overhand rods and casting here in the northeastern US saltwater scene generalize all rods with the extended lower handles of longer length as spey rods. They are generally not concerned with a rod's spey casting ability, looking specifically for rods with overhand tapers and rarely if ever utilize any spey casts thus IMHO are incorrectly referring to 2-handed rods as spey rods.
I am working on a two-handed overhand rod with Nobuo, it is not a Spey rod... but when completed will be a heck of a two-handed rod for the northeast coastal fishery and perhaps other similar fisheries around the world.