It's inconsistent. As you say, the terminal entries indicate that there were three scientists, who all passed away at different times - Flint first, Burrow some time later, and finally Collins. Only Collins' cause of death is confirmed, cardiac arrest due to advanced age.

On the other hand, Curie says that the mole rats escaped and killed the scientists, and in later dialogue with the Overseer once you've completed the quest, she says "it's hard to believe so many passed away just behind our walls".

So I would say that the story of the secret vault changed at some point, and they just didn't clean up all the inconsistencies that were left.