Obligatory I am not a lawyer, this is just my opinion.
A let’s play is a derivative work. You can claim fair use, but that’s hard to do. Fair use often boils down to a question of ‘does the derivative work compete with the original enough to cause a loss in sales?’ Think of when people film themselves watching a movie for YouTube, without cutting anything out and barely commentating over anything, meaning that someone could watch their video instead of the movie and get almost the same content.
In this case, he filmed himself playing the entirety of a visual novel. I think it’s fair to say that for a lot of people, his let’s play could absolutely substitute for playing the game, thus losing sales for the developer.