Oh FFS, java2d is hardly bare metal, and there's more going on "under the hood" with it than with anything over OpenGL. In fact, Java2D's abstractions are the problem when it suddenly decides to keep your images in main RAM or just use a software renderer altogether and you can't do jack about it.
It doesn't get any more "under the hood" than writing a shader.
In any case, he's chosen his scope, and whether you like that scope or not, he's doing a far better job of explaining these fundamental aspects of game design than most Youtube tutorial makers. I'm just letting people know that this exists, because I personally am finding it to be a helpful introduction. And that's all he's designed these videos to be. Criticizing them because he's not using the external libraries of your choosing is totally counterproductive, especially if you're interested in nurturing the Java game design community. Beginners can learn external libraries whenever they want. Understanding fundamental design is a good way to start the larger process.