I think you’re missing another big factor here: being within 3 feet of a single person for 2 hours is VERY different from being within 3 feet of a different single person, every 5 minutes, for 2 hours. I believe microCOVID actually maxes out the risk of catching COVID from a single person in less than that amount of time.
It is very different if you have high transmission probability per hour, but not if you have low.
For example, if I had a 50% chance of catching covid from a 15 minute interaction, conditional on the other person having covid, then if I spent 2 hours with that person, my risk still can’t go above 100% times the chance that they have covid. On the other hand, if I spent that 2 hour period with eight sequential people, my risk can be up to eight times higher.
Right now, the tool estimates that if you spend an hour with a single person indoors who has covid, masked and vaccinated, the chance that you get covid is about 0.1%. This is low enough that the difference between one person continuously vs several seriously is not significant.