This is true, but that's not what I call "conspiracy theory". To see the difference, take Chomsky (or Assange, to an extent) as a good example. They are very critical to U.S. foreign policy, yet they never claim something without evidence (they don't claim for example that 9/11 was inside job), and as such they are not conspiracy theorists.
The problem with people that have conspiracy theories is that they ultimately defend same or worse atrocities, but done by some other party or even themselves.
> And that figure of 10 000 people can't keep a conspiracy secret is predicated on the idea that it's some kind of active plot
It's just a rule of thumb, where you should set a canary and deeply rethink your beliefs and evidence you have for them. You see, in order to maintain belief in conspiracy theory, you have to categorize all contradicting evidence as coming from either co-conspirators, or people who are "naive" and somehow manipulated. This means that over time, number of people that have to be involved in conspiracy will grow.
Of course there can be conspiracies of 10k people or more (e.g. NSA), but they are very difficult to keep secret in practice (there was on average one whistle-blower per 2 years on NSA spy programs). There can also be situations that look like conspiracies on the surface, as you correctly state, but those don't have the property that they can actively suppress dissent (which is required to sustain belief in conspiracy theory, to reject the contradictory evidence).