The video is a Tool-assisted Speedrun, and he who posted it has stated that it is. His videos are more than 10 years old, and some of them may have been executed faster than that TAS due to new discoveries, but it would be difficult to do that in this game...

Hello, I'd like to request leaderboard moderation for Milon's Secret Castle. I have a pending run, but the moderator hasn't logged in for about 8 months and there is no stream on his twitch, so his activity couldn't been confirmed. Thanks. https://www.speedrun.com/milons_secret_castle