Reservoir bag
The purpose of the bag used with the mask is not rebreathing CO2.
It is a reservoir for pure oxygen, and if the mask is working as designed, no exhaled gases should enter the bag.
Inspiratory flow rates exceed 10 l/minute and comprise about 1/3 of respiratory cycle, the reservoir "traps" the flowing oxygen during exhalation and makes it available for the next inhalation. Otherwise, 2/3rds of flowing oxygen is simply lost into the cabin. To inhale (mostly) pure oxygen from a mask without a reservoir bag would require flow rates that would deplete most systems in short order.