[Edi]

The reason it's a normal battle report is that if the defender wins against the forces stroming the fortress, it is shown as a normal battle report for both sides. If the attacker wins a siege, he gets the "We captured the fortress!" and the defender gets a normal battle report.

Since the outcome was that the defenders won (and we have a wonky replay that visually shows the opposite), it's handled as a case of the defender winning and the battle reports accordingly.