But everyone thinks that only two people (they and I) know and nobody tells. So the entire population knows something, but they don't? I don't know that's why I'm asking, it just popped into my head haha.

But you see thats also saying that they still keep that they know a secret so if each person tells another person they would soon find out that everyone knows therefore neither thing is really a secret anymore
+1 answer in: “Less say I have a secret and I tell everyone on this planet, one by one and covertly. Everybody knows the secret but only I know that, the rest of the population think it's just me and them. Is it still a secret?”