I don't think he is making the assumption that they know each other.
Each individual has a tolerance of what they can comfortably cope with. If 3^^^3 people were all experiencing a pain that is below that tolerance, nobody would be prevented from happiness. However in the other situation, the tortured individual clearly would be.
That's an example of infinite or unbounded utility functions: no matter how many specks of dust in the eye, it will never add up to a single person being tortured for 50 years. Even 3^^^^^^^^^^^3 specks of dust. Unfortunately the mathematics of infinite and/or unbounded utility functions doesn't work out well. It leads to some seriously messed up edge cases. (So does finite utilitariansm, to be fair -- [Pascal's mugging](http://www.nickbostrom.com/papers/pascal.pdf) -- but these are fully dealt with by decision theory, whereas the infinite or unbounded cases are not). It's not very strong, but it is evidence that we should be accepting of the calculations of finite utilitarianism since the formalization works out better in cases which are within the realm of our experience.
