Big guesstion, do they have one, or two engines? A combiner gearbox does not fare well in aviation, as history show.

I have a hard time believing they can get to such speed with just 500hp.

That advertised speed seems to be ground speed, the trick is that they designed this thing to fly very high where the corresponding IAS is much lower. What's new (and a truly surprising as impressive technological achievement!) is that they do this on a piston engine which are notoriously less capable at high altitudes than jets or turboprops.

An implication of this is that short-haul applications will see neither the speed more the mileage advertised. I still want to see the general design principles applied to the problem of regional feeder flights, it won't be quite as over the top impressive but it would still be the closest to "slow steaming" available

(speed really doesn't matter on feeder connections where boarding alone takes longer than the flight, but it's very hard for aircraft to trade speed for efficiency once non-negligible payloads are involved)

One engine, a turbocharged V-12.

And as others have mentioned, it's taken advantage of newer technology that allows six of pistons to be turned off.

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.

Naturally aspirated != carb.

You can have a naturally aspirated engine with electronic (even direct) injection, or a turbocharged engine with a carburetor, or any other combination.

True. Naturally aspirated simply refers to the lack of a turbocharger/supercharger pressurizing atmospheric air as "boost".

I just wish carbureted was an easier word to spell after not using it for months at time.

which pretty much keeps it restricted to overland only. it took a long time for the FAA to permit twins to fly across the ocean? ETOPS (Extended-Range Twin Engine Operational Performance Standards) is just what it says, twin engine but is there any possible means a passenger carrying single engine could get certification? Now as an unmanned cargo drone it might get this permission.

You can fly a single over the ocean, you just have to carry extra equipment.

Or compare it to a single engine very light jet like the cirrus visionjet.