If the moon landing was indeed infeasible from a technical perspective at that time, those people would know, that their designs were inadequate or flawed. The "some people might think they do legit work" argument doesn't really hold, because they would either know it was an impossible feat, or it would possible and no need to fake the landing.

The movie The Cube springs to mind. No one knows what anyone else is building other than a very few who put the whole dastardly thing together.

https://en.wikipedia.org/wiki/Cube_(film)

Even faking the landing would require cooperation from parties with nothing to gain, since independent parties were able to bounce a laser off the mirror that they placed when they landed.

> Not a skeptic myself, but the argument that 400k people needed to keep a secret is pretty weak.

That's unless you start looking closer. Those people, they aren't universally fools, they think by themselves. Maybe not all 400k, but a sizeable non-reducible chunk of them do.

It could be a half lie too. 400k people built rocket, they saw the launch but the rocket didn’t actually make it to moon. The actual moon shots were in a studio.

Half lies are the best lies because you can argue half a truth is good enough for the whole truth.

The argument still holds because you'd need to keep 40K people silent - which is impossible.

There are single individuals that even the President of the US cannot keep silent.

A democratic country has no ability to keep monumental secrets like this.

That still leaves 40,000 people who are in on that lie.