It's not an April Fool's joke; it was an intentional effort to deceive the competition to highlight the fact that such competitions aren't well-prepared for the addition of AI to the artist's toolbelt.
The artist refused the award, and declared it was their intent to highlight this concern.
What efforts like this are missing is publishing a cryptographic hash before engaging in the deception. Otherwise it can seem like the author intentionally decided to mislead but might have changed their intention after winning.
Publishing something openly like e1c7aa9150766bd5279b54951e3828ed9c19367af21f03f669eb586bd54c13b1 which can later be verified once "revealed".
$ echo "I, CapableWeb, is gonna participate in Sony World Photography Award 2023 with a AI generated image in order to see if they are ready for AI generated images. 2023-04-14" | sha256sum
Don't really have to involve blockchain here (yet again), publishing on a website and letting Internet Archive archive it is good enough. Or even posting it on Twitter/HN/$social-media-with-edit-limit
The artist refused the award, and declared it was their intent to highlight this concern.