In the interview of MR. Kojima made by Benjamin Kratsch, the german journalist has made a mistake in his english/french translation.
The right answer in the interview was this :
Kojima: There could be a Next Metal Gear with the new fox engine, but what I showed was no ingame or something like that. It was just a test case for us, that we’ve build internally and then going public because the lighting and all those interaction with the environment when the horse is walking down there and the plants and foliats are moving around the back oft the horse was just wowing everyone.
But in the French version Benjamin has sent us the english word "could be" has been translated it into "will be", which means more or less MGS 5 will be based on the Fox Engine which Kojima never said. Since we didn't have the english version, we haven't seen the mistake.
We all apologize for that error.