The CO2 is also an input; it originally comes from the environment, so this is net-zero emissions in the same way that biofuels are. From the article:

   Although the bus emits CO2, Team Fast argues that the original CO2 used to create the hydrozine is 
   taken from existing sources, such as air or exhaust fumes, so that no additional CO2 is produced
    - it's a closed carbon cycle in the jargon.