Even in the show most don't die immediately. Many of the workers are shown being interviewed about what happened before they die in a hospital very far from the reactor. Days/Weeks passed.
That, and besides, the number of deaths related to Chernobyl (or even the magnitude) is highly controversial. But even the "official" Soviet number of direct, short-term fatalities is not 2, but 31. And I would take that number with a big grain of salt, for the very same reasons depicted in the series.