Anaerobic, aerobic, and what the words mean
Every coffee ferments. The only question is what has access to oxygen while it happens.
After picking, a coffee cherry is fruit wrapped around two seeds. Between the two sits mucilage, a sweet sticky layer that has to come off. Removing it is fermentation, and it happens whether anyone plans it or not.
Aerobic fermentation happens with oxygen present: cherries or wet parchment in open tanks, channels or on beds. The microbes that do the work are the ones that thrive in air. It is fast, familiar, and what most coffee has always done.
Anaerobic fermentation happens in a sealed vessel with the oxygen excluded, usually a tank with a valve to release gas. A different population of yeast and bacteria takes over, works more slowly, and produces a different set of acids and aromatic compounds. That is where the intense fruit, and sometimes the boozy edge, in modern processed coffees comes from.
Natural and washed describe something else entirely: whether the fruit is still attached while the coffee dries. A natural dries inside the whole cherry. A washed has the fruit stripped first. The two vocabularies get combined, which is how you end up with a term like anaerobic natural.
This lot is one. Luis Anibal collects only ripe cherries, ferments them dry and sealed for 25 hours, then moves them to his parabolic dryer for an average of 25 days. Slow, covered drying matters in Huila, where humidity works against anyone trying to dry coffee inside its own fruit.
None of these words are quality claims. Anaerobic run badly tastes of vinegar and nail polish. The process only tells you what was attempted.