The Emperor might have encouraged discourse once and even allowed the subsequent words by Creten to influence his decision but that was the limit of his accommodation.
He ordered them to leave and made it very clear that his decision in regards to the duel was final and as they walked out of the throne room, neither Tanya nor Creten was happy.
Creten was unhappy because he was clearly at a disadvantage and all the points he had previously brought up about how he would surpass Tanya were all in the future. He had no assurance of victory right now!
Tanya on the other hand was upset that she still had to prove herself but she was much more readily accepting of it all... And why not, she had even more of an advantage than her brother knew.
...
Not long after the two Imperial siblings left the Throne room to prepare for their duel the next day, a message came for the Emperor and as it was delivered, Amador's eyes widened in surprise all the while.