"She was really strong," Callius mumbled to himself inside the dimly lighted training room.
At that point, Diana had already left the training room as she wanted to take a rest and rearrange her thoughts. In a way, even though Diana was a peak 4-star mage, she was not a person that had the body of a 4-star knight. Therefore, she left the room soon after the spar ended as she was tired both mentally and physically while Callius stayed inside the room.
This might make her slightly suspicious of Callius' real power, but since she was Diana, Callius didn't want to be overly cautious, as he knew she was not someone that would reveal such a simple detail to others, especially when it was about him.