We're talking about using features exposed by the OS (the Core Animation layers API in this example) to avoid doing unnecessary work to save battery life.

Any browser could adopt each individual platform's proprietary graphics APIs, but using the same API everywhere (traditionally OpenGL) is less work.

Any browser engine is free to do the work to use per-platform APIs. If your argument is that cross-platform browser engines won't do the work to optimize with platform-specific APIs but that Safari will, WebKit is a cross-platform browser engine and so it adopting CoreAnimation on Apple platforms in addition to supporting other platforms is exactly the same amount of work that it would take for Blink or Gecko to do the same thing.

My argument is that they don't do the additional work, resulting in their energy efficiency deficit.

As pixels per inch and screen size increases, the cost of doing unnecessary rendering work becomes more noticeable.