I find it quite obvious that the only reason Nintendo was there, was because Spike wanted them to be there. Geoff kept on hyping that it was going to be a World Premiere and something very special, while he probably knew that it was going to be Donkey Kong. The schedule itself said that Nintendo was going to play a demo of a game, which rules out most other games.
Spike must have asked Nintendo last minute and then started to hype it up, in order to get Nintendo fans to view their show, and Nintendo just did what they could do in such short notice, which is showing a soon to release game.
The fact that everyone seemed to be hinting at Zelda probably got a lot of people excited, which probably led to an increase in the audience, so in other words, Spike's plan worked.
Next to that, Nintendo got a lot of time to talk about their already shown game, while other devs got quite little airtime to talk about their games. This kind of shows that Spike did everything they could to get Nintendo on their show (in my opinion).
