If Y and Z are known to not be X and not to be related to X, that's one thing. So are trolls who just deliberately derail things. If A, B, and C are unknown and can be shown to be equal or equivalent to X, then the thread is still about X because being about X implies that it is about A/B/C and vice versa. However, even if A, B, and C are shown to not be X, the determination of that is still about X. Addressing the question of definition when ambiguity is present - considering only the reason of a lack of specificity within the thread and not that the person posting has not taken the time to familiarize themselves with all of the information previously presented within the thread - is entirely valid. Since this ambiguity will almost certainly be present given the vagaries of natural language communication, this seems like what you are saying is that "Because there is ambiguity in the definition of X, examples of things which are not X arise in a discussion about X."