That's not a very good choice of words but I guess you could say it is consuming the alkalinity hardness.
It doesn't literally 'consume' the KH, by producing CO2, the CO2 is using up, converting the available buffer, making the pH become more acidic. When the acidity reaches a certain level...
The nitrifying bacteria consume the ammonia, converting it to nitrite and nitrate ....not KH. dKH is degree of alkalinity or carbonate hardness, which is related to pH. During the process, acids are produced, lowing the pH. Adding baking soda (calcium bicarbonate) to the system, your increasing...