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That’s a super simplified model of the world. Investors put a lot of work into finding out information between earnings calls, including talking to suppliers and customers, acquiring data on sales (obtained via retailers, credit card companies, satellite data, etc), and observing external events that affect the market including regulatory changes and the earnings of peer companies. All of these bits of information can provide insight on the hidden information.

There is, of course, speculation in the market. My objection was to use of the phrase "pure speculation."

As Benjamin Graham said, the stock market is a voting machine in the short term but a weighing machine in the long term.

Not just earning calls provide new information. The market also reacts to economic data, key people leaving a company, regulatory changes, natural disasters and so on. If it changes the company's outlook, it will also move the stock price.