Since A and B are only random subsets of all possible combinations (in the case of integration testing), they are not going to detect all possible unknown bugs

The key lies on C, the common area. If you look at the Venn diagram and if you imagine squeezing the superset T smaller and smaller, it will be less likely for C to be small--A and B must tend to overlap.

The whole point is to estimate the total number of bugs without having to go through an exhaustive testing.

Of course, that simple estimate probably won't be statistically very valid, since bugs are not independent. But it still gives a good conceptual insight--if a bunch of independent testers tend not to find common bugs, there're probably still pretty of bugs out there.