If he really feels this way, why not say something like "The full demo could have spoilers that affect your enjoyment of the game when it is released. There will be a gameplay trailer cut from the footage released afterwards that avoids spoilers for those who want jump in completely fresh."
Also, there's been what? 30 minutes of cutscenes released for the game? Yet it's the gameplay footage that he's telling people not to watch.
Kojima is a great developer, but he really has a massive ego. This brings me right back to the whole "Don't trust early Destiny reviews" fiasco. That game was from a developer with a great history and it was a mess at launch.