We may use a fictional truth to construct a logical deduction, otherwise known as a proof. Basically, any number of fictional truths may be appended as assumptions to a deduction to form a new deduction. The deduction itself is valid and perhaps useful or interesting. No one can deduce with logic alone that the fictional truth is true, but that should not discourage anyone from speculation.
Cartoons are examples of long deductive sequences made up of mostly fictional truths.