This argument appears to be invalid: even if (3) and (4) are true, (5) could be false. If Alex is not aware that Hesperus and Phosphorus are the same planet, then it seems that he could believe that Hesperus is visible in the evening while rejecting the claim that Phosphorus is visible in the evening (perhaps he thinks Phosphorus, the morning star, is only visible in the morning). The principle that coreferring names are substitutable "salva veritate" thus appears to fail in the context of belief reports (and similarly for other propositional attitude reports).