People got post-research interviewed and asked to explain their answers. There were social feedback mechanisms. Even if there wasn’t peer to peer social feedback, it was certainly possible to annoy the authority (researchers) who is giving you the questions (like annoying your teacher who gives you a test). The researchers want you to answer a particular way so people, reasonably, guess what that is, even if they don’t already have that way highly internalized (as most people do).
This is how people have learned to deal with questions in general. And people are correct to be very wary of guessing “it’s safe to be literal now” (often when it looks safe, it’s not, so people come to the reasonable rule of thumb that it’s never safe and basically decide (but not as a conscious decision) that maintaining a literalist personality to be used very rarely, when it’s hard to even identify any safe times to use it, is not worth the cost). People have near-zero experience in situations where being hyper literal (or whatever you want to call it) won’t be punished. Those scenarios barely exist. Even science, academia or Less Wrong mostly aren’t like that.
More related to this in my followup post: Asch Conformity Could Explain the Conjunction Fallacy