People in this thread are implying that a coin toss relies on chance. This is wrong. It relies on the force exerted on the coin, where it was exerted, how far the coin has to fall, what surface texture it is falling on, the density of the air, the mass of the coin, the shape of the coin, the size of the coin, the width of the coin, etc. There's lot of variables, and most will change every time you flip that coin, and no person on earth is capable of measuring and calculating the exact result. But that doesn't mean that it's up to chance because you can't possibly hope to work it out.
Same goes for diplomacy. If you knew everything about a person down to what they ate, the composition of their brain, the number of brain cells they have and their individual function, all of their nervous impulses, etc. Essentially absolutely everything about a person, including things they don't even know, you could predict everything they did up until the point they interact with something outside of your prediction.
If you had all the data of every single sub-atomic particle and light ray, and were capable of processing it faster than the universe can, you could predict everything that happens in the universe with 100% certainty, and that includes what moves your opponent will make in a game of diplomacy.
Nothing relies on chance, ever. That said, probability is a good thing to know when you're not capable of knowing everything.