For anyone curious about the platform and release date being off, I bought it on the Amazon Appstore, which did release on this day as intended, to play on Android.
This final session was the only PS5 session. It is still marked as "Played on PS4" since I don't think there's a noticeable performance difference between the consoles for Undertale specifically. If I ever decided to play again, it will be marked as "Played on PS5"