If the ATM offering is priced at the current share price, and the company plans to just sit on the cash that it raises, the share price won't move much. Reason being: even though your shares are diluted, the company's total valuation increases (bc of the extra cash it just raised). Smaller piece of a larger pie == same value.

However, AMC's offering was far from this ideal scenario. Instead of sitting on the cash raised, they're burning through all the cash they're raising just to keep the lights on. So, shareholders experienced dilution without any increase in company value. In a sane market, you'd expect share price to decrease by the dilution percentage. The fact that this hasn't happened (combined with the fact Cinemark's share price is still half of its pre-pandemic levels) should worry anyone with a long position.