If you can conceive of a scenario in which X is true and Y is false, this demonstrates mainly that X and Y have different \[intensional definitions\] inside your own mind; you have separate concepts in your mind for X and Y\. It doesn't demonstrate that a world where 'X but not Y' is logically possible, let alone physically possible, or that X and Y refer to two different things, etcetera\. Whether 91 is prime or compositive is an immediate consequence of the definitions of 'prime' and 'composite', but it's not literally and explicitly written into our brain's representations of the definitions of 'prime' and 'composite' that 91 is a prime/composite number\.
This paragraph is hard to understand. May be rewrite it in a more concrete way, i.e. using "is 91 prime" as an example from the start.