From the book description it does say the singers initiate a dangerous operation that leads to an arms race and unlocking secrets about the tower. An attack on Urithiru may make some sense from this paragraph though I am not positive.
Quote below
Now, as new technological discoveries by Navani Kholin’s scholars begin to change the face of the war, the enemy prepares a bold and dangerous operation. The arms race that follows will challenge the very core of the Radiant ideals, and potentially reveal the secrets of the ancient tower that was once the heart of their strength.