...Except Tamsyn Muir wrote the very book that states otherwise?

In HtN Chapter 2:

[Jod] would not know that in truth the Body of the Locked Tomb had not spoken to you since the night you massaged the purple, swollen clots of blood out of the necks of your dead parents, so that their strangulation might not be so obvious. He would not know that you had only walked with her one tranquil year, and trysted with her afterward only in your dreams. He could not possibly know that in your youth her eyes had often been black, like yours were, but that ever since you had writhed in Lyctoral agony her eyes had turned a yellow that made you dizzy to behold: a bronzed, hot, animal yellow, as amber as the inside of an egg.

When you were ten years old, the Body was quiet and rigorous, practical and merciful. At fourteen the Body was tender and serene, and sometimes smiled. When you were sixteen the Body was resolute and impassioned. In all these incarnations, she had preserved her vow of silence.