Nomura stated that, not me, lol.
I don't think however that Mickey used the darkness. Besides Aqua, Ven and Sora Mickey is the character shown to use the most light aligned stuff.
What these similarities between Riku and Mickey are or what causes them is unknown, it's only said that they both aren't that much affected by Darkness.
Certainly plausible, although I wouldn't say that the KK D is the reason for Mickey's good resistance against darkness. He walked the Realm of Darkness quite some while before Riku's heart was cast down there and reached the realm by using a friggin' randomly popping up Corridor of Darkness in Traverse Town (as was explained in Coded).
Oh, and thanks for sparing me the task to search for the interview snippet. ^_^
It think the second case is more likely, as if only the KKD would cause that immunity it wouldn't be a speciality of Mickey but of the weapon, which would render the statement that Mickey has similar resistance as Riku false.
The correct statement would then be: The KK D grants the same(or similar) resistance to darkness as Riku developed.