The shot does not kill the fin whale instantly.

Source: https://www.dailymail.co.uk/news/article-12218705/Fin-whale-puts-two-hour-battle-survive-shocking-video-blasted-harpoons.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

When whales die, their guts begin to fill with methane gas, which often causes the corpses to inflate like a balloon, float on the ocean's surface — and occasionally float onto shore like the dead fin whale.

Source: https://boingboing.net/2023/07/14/a-dead-62-foot-whale-spooked-examiners-when-it-made-bubbling-noises-a-warning-it-could-explode.html