Well this is thoroughly horrifying! Here’s the explanation:
Because this is fresh squid, the cells inside itare all still intact. The biochemical machinery still functions. There is still a source of energy for the muscles in the form of unused ATP molecules stored in the cells. All that the muscles need is something to activate them and they can still contract and relax (until they run out of ATP or something else shuts down the biochemical machinery). Sodium can be used for this, like that contained in the soy sauce.