Do I have to use sawdust on poos?

Yes, or something equally absorbent. 

Urine diversion is not 100% effective and poos can be loose/runny leading to the build up of raw sewage in an open container…not pleasant, or safe. This is why you must keep the solids tank as dry as possible, very easily done with the magic or sawdust.

After each ‘deposit’ put a handful of large sawdust on top which absorbs any moisture, the idea with a compost toilet is to separate the liquids from the solids and keep the solids as dry as possible which is what keeps it smell free.