You asked, "...can’t you have some random bacterium in your sputum due to eating or mild gerd, and not necessarily MAC..." and the answer is "Absolutely" Often it is noted as "Normal flora" or "clinically insignificant [whatever they find]" Once my results even read "random artifacts including [some unusual bacteria in small quantity]"
Remember, our noses serve to trap bacteria, pollen, etc and keep it out of our lungs - and often sputum can be contaminated with mucus from our nose/sinuses.

This is why it is a good idea not to panic whenever we read preliminary ;ab results. It's important to wait for the doc to decide what is meaningful.
Sue