The engine basically sounds like two in-line six cylinder engines that share many common single engine features but with the ability to shut off one side and use power from the remaining side.

So hypothetically, you fly up to altitude and climb using all 12 cylinders, then cruise on six.

That could be where the fuel efficiency is being notched up versus burning all 12 cylinders at all times.

Kinda like how people are still amazed variable displacement engines took them from 10mpg to 18-20mpg. It's also likely the engine has a number of other more recent innovations included too. Variable valve timing, etc, etc. A lot of new rather exciting new engine technology has come out in the last 20 years.

It's much like when we went from naturally aspirated aka carbs to electronic fuel injection/ignition.