I believe there's truth to this. Nintendo has been very resistant to using King K. Rool and other elements from the RARE Donkey Kong games unless they absolutely must. Getting K. Rool into Smash Bros was very much an uphill battle. Right now, they prefer to emphasize the Retro Studios iterations of the characters.
There... is a villain, though. Tiki Tong is all but confirmed to have a scene on the ride.
Beyond that, though, Nintendo has always been weird. Most people would assume/want Mario Kart to be a fast-paced thrill ride -- and Universal Creative pitched versions of that -- but Nintendo said no. It's just how it is.