It's actually potato starch.

Sawdust isn't just cellulose. Wood is a matrix of cellulose fibers bound together by a tougher compound called lignin. While lignin isn't particularly toxic, you wouldn't want to eat it.

Fun fact, a lot of the process of making paper is for removing the lignin, only the cellulose is useful for making paper. And grasses like bamboo have a higher ratio of cellulose to lignin, so it's better for making paper