Ok, let’s say 20 people get exposed to Ebola the same day. At Day 21, none are showing symptoms. There’s a high probability that one of them will get sick from Day 22 to Day 42.
With enough exposures, 21 days won’t work.
The statistics (admittedly from Africa) indicate that each person who is exposed should get sick by around day 6 or 7. If there was an exposure for a group of people and none of them got sick at day 6 or 7, then makes it less likely that any were infected in that event (see hypothetical event below). But all exposures are all individual. How much virus each person got depends on how much vomit they touched, how much virus was in that particular vomit, etc. The incubation period does not shorten or lengthen based on that factor. They either got infected or not, and they will get sick or not around day 6 or 7 and most likely by day 21 if they were infected.
Hypothetically If 20 people were exposed at the exact same level (impossible to know or constrain) and none of them were symptomatic at day 21, then the odds are essentially zero that any of them will get sick after that because there are extremely high odds that one of them would have had symptoms by then.