The statement/claim "10,000 People Can't Keep a Secret" could be best described as hyperbole used to make a point; the point being the more grand the conspiracy, the less likely it is to be true.
So when people say X number of people can't keep a secret, they're ignoring the fact that people who DO reveal secrets are often ignored.
People who present
good evidence are rarely ignored. Conspiracies have been and are exposed every day by whistleblowers who come forward to present evidence.
It is any journalist's/media outlet's dream to expose such a conspiracy. Similarly, a scientist who presents evidence that leads to a paradigm shift would win a Nobel Prize (e.g., showing that the earth is flat, God created the universe 6000 years ago, evolution is bunk, climate change is a hoax, vaccines cause autism, etc..). The problem is, people often present "theories," personal testimonies, and really bad evidence (i.e., "Of course there are aliens in Area 51. Why else would it be so protected by the government???" or "I was anally probed by an alien so I know they are real.").
Rationality includes making probability calculations and weighing evidence. It is a fact that there is a strong correlation between the number of people who are asked/bribed/threatened to keep a secret, the more likely it is that the secret will be exposed. The testimony of the 80-year-old former NASA employee that "he saw a UFO" has to be weighed against the number of NASA employees who
never made such claims. Statistically, we know a portion of the population is crazy, liars, starving for attention, etc. When we someone comes forward claiming to expose a conspiracy, and has no good evidence,
the rational thing to do is to not accept their testimony as fact until evidence supports the claims being made is presented. There is a saying "the plural of 'anecdote' is not 'evidence'," which means that a bunch of people presenting bad evidence doesn't make good evidence. However, in some cases, more corroborating testimonies does make for stronger evidence. An example might be 30 strangers in a park of 200 people reporting simultaneously to the police that some guy ran through the park naked. The idea that 30 strangers made this up out of a population of 200 is far less likely than the fact that some guy really did run naked through a park. Situations and details matter.
A good skeptic must remain skeptical without being cynical, that is, they must not be dismissive yet know how to evaluate evidence, which often requires specific knowledge as well as general knowledge of statistics, sciences, psychology, and more. However, a good skeptic can't also adequately investigate every theory they are presented--
it is temporally impossible . This is why the maxim "extraordinary claims require extraordinary evidence" works well.