SEC rules aren't the only concern, and "market moving" was perhaps too specific a phrase. There are lots of legitimate legal and financial reasons that a company may choose to be circumspect in its communication.

As one totally hypothetical example, let's say that the project a candidate is being interviewed to work on is suddenly in jeopardy. This could be due to sudden unexpected financial hardship, like a big contract falling apart at the very last minute. Or it could be due to a new competitive threat resulting in a need to shift resources elsewhere. Or the new CEO just doesn't think it's worthwhile. Or, maybe the project ultimately proceeds and the candidate gets an offer.

In any of these situations, a company can easily find itself the target of an investor lawsuit if the reason for the hiring delay is inadvertently disclosed and there is some ensuing stock movement, even if no SEC rules are violated.

You might still think that this is overly cautious, and you may be right. Either way, ghosting doesn't only happen because people are inconsiderate (though that is certainly a reason), and I personally try not to infer too much about the culture when it happens to me.