Solving Schmitt trigger circuit using the superposition theorem
The Wiki article is using superposition correctly. It starts by assuming the output is in one state, then assuming the output remains in that state, calculates the comparator input with respect to the circuit input. It uses this relationship to determine the input level (producing zero volts at the comparator) to determine the trigger point at the specified state. It then does the same for the other state. What it does not do is to model the behavior of the circuit as a trigger point is passed - it only identifies what the trigger points are.