CO2 and ammonia are not directly related. Fish produce waste matter, which contains ammonia. Rotting food, plant matter, dead fish, etc. also produce ammonia. None produce much, but enough for algae spores to detect. CO2, on the other hand, is largely introduced to the water externally, almost entirely externally in an aquarium. Variations in concentration of CO2 in the water, in nature, signal that something is changing - seasonal changes, for example. But, algae spores apparently can pick up those changes too. All of this is survival tactics for algae, "learned" by selective survival in larger numbers by algae that are good at picking the best times to bloom, thus producing the most "offspring". And, all of this "learning" took place long before any of us began the aquarium hobby.