Two definite expressions cannot always be interchanged without changing what the utterance communicates. If a hearer knows Zayd and needs to learn whether Zayd is his brother, «أَخُوكَ زَيْدٌ» will not answer exactly the same implicit question as «زَيْدٌ أَخُوكَ».
One order follows the hearer’s established topic. Reversing it establishes another topic and a different information focus.
Thus semantic correctness and fitness to context are distinct requirements. An utterance may be grammatically possible while failing to give the information the particular listener needs.