Glucose, which is pretty similar to cellulose in composition, does indeed contain two hydrogens and an oxygen for each carbon. But the carbon has a mass of 12 AMU, the oxygen of 16, and the two hydrogens total 2 together. So 6⅔% of the cellulose is hydrogen by weight, and the other 93⅓% is carbon monoxide.

(I have no idea whether the oxygen atoms from the water end up in the glucose or in the excreted O₂.)