Well, the Matron's Lethe is a root, and it was stated that it was formerly buried underground only to be revealed by the massive land upheaval caused by the Calamity, so, using the etymology of the word 'lethe', maybe the 'lethe' part refers to the fact that the tree it was formerly attached to is gone, but the (formerly buried) root remains, a literal 'forgotten memory' now revealed? Or maybe I'm just clutching at straws here.