Mrs. Dugu was a smart woman. She and Feng Wu weren't archenemies. There was no need for her to drag herself into things.
She gave it some thought and said carefully, "There's no direct evidence to suggest that you were the one who wrote the letter."
Old Mrs. Gu glared at Mrs. Dugu.
Mrs. Dugu said, "But it's certain that the letter was written by the same person who wrote that schedule."
Feng Wu said, "Are you sure?"
Mrs. Dugu said, "Absolutely!"
Feng Wu chuckled. "But I didn't write the schedule."
Feng Wu was incriminated because of the letter.
It was believed that she killed Mrs. Gu because the lady had walked in on her secret meeting with Jun Linshen.
The logic would only work if Feng Wu had really written the letter.
But now, she said calmly, "I didn't write that schedule. Therefore, I couldn't have written the letter."