Review embargoes are a pretty well understood concept.

Sometimes the review embargoes don't end until the game release or just before it. Not this was a review so that wouldn't matter anyway this was just directly recording the content which is always going to be illegal.

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.

Putting it in the simple most non legalese way possible.

To go to the trouble of pressing charges or suing you have to show "damages" (Physical, financial, emotional) and official reviewers almost always have to sign "embargo" paperwork telling them explicitly what they can and cant talk about before a certain date.

By putting this content out before release, he potentially impacted sales by a noticable amount and likely either obtained the copy illegally, unethically or broke contract to do so.

They can likely show damages and point to breach of the law in doing so.

Exactly. They own the game.

Let's plays are derivitive work. They legaly can do whatever they want with it.

Mojang also already DMCA'd people: https://youtu.be/9ARAhvFEA3s