Ok, things are starting to make sense.
I guess Donkey Konga is definitly the Namco game since Namco has made some drum games before. What probably happened is Namco was already planning a new drum game and simply convinced Nintendo to allow them to use DK characters in it to help it sell. Nintendo itself is still of course making the "2D meets 3D" DKC platform game that Miyamoto mentioned.
And, come to think of it, there already was an updated version of the original DK arcade game shown at E3 LAST year which I'm sure will eventually be released. If I recall correctly, it was actually a GBA game and simply used the Gamecube through the connectivity feature to design levels.
Maybe Nintendo will just convert it into an entirely GCN title and include it in a collection with Miyamoto's rendition of Pacman or maybe as a pre-order bonus for one of the bigger DK titles or something.
So, yeah, that's three entirely different Donkey Kong games we have to look forward to.