We have here a problem in nomclature. The same name applying to two different engineering designs.
It is obvious that the writer of the article linked to has no idea of the existence of Fairbanks-Morse and Deltic OP engine designs. He thinks it is a new concept. Maybe the "ship engine" he referred to may be an FM (used mainly in submarines).
I know of no fighter plane to ever have used this design, although "opposed piston" engines of the Subaru type are commonly used in light aircraft. These are "boxer" engines, like in the old VW Beetle or the Corvair. A flat crankcase is fitted with two or three cylinders on each side (usually air-cooled). The pistons are 180 deg. across from each other.
The FM and Deltic engines are two-cycle Diesels. To come up with a four-cycle engine would require some innovative valve design (like the sleeve valves mentioned).
A boxer engine uses only one piston per cylinder and only one crankshaft in the center. It is just another way of arranging cylinders in a conventional four cycle engine. A two cylinder Briggs and Stratton garden tractor engine is another example.
A true OP engine uses two pistons in each cylinder, coming toward and away fom each other in the center of the cylinder. Multiple crankshafts are used. In the FM engine, the cylinders are arranged in a row vertically. There is a crankshaft at the bottom of the engine, connected to the lower set of pistons, and another at the top of the engine, connected to the upper set of pistons. In the Deltic engine, the cylinders are arranged in rows of triangular sets (deltas). Each corner of the triangle holds a crankshaft. Each crankshaft is connected to two rows of pistons, each row of pistons in two of the three sets of cylinders.
If you can follow this description, you can see there is no place to mount conventional poppet valves. So OP Diesels are two-cycle, using forced induction and ports in the cylinder walls.
With two pistons in each cylinder, the power output of a given number of cylinders is effectively doubled over a conventional Diesel. Although OP engines are by necessity quite large, they still have high horsepower output for their size.
The obvious drawback to OP engines is maintenance time. To pull a piston, an entire crankshaft must be pulled. Also, due to the movement of pistons in ported cylinders, they are notorious finger-eaters.
The OP engine is such an efficient design, that some in fixed-power applications have run literally for decades without being shut down. Stationary engines can have their lube oil and coolant changed out while the engine is running (think electric generating plants).
If a true OP gasoline engine can be developed, it would certainly put a lot of power in a small package, but it would still be larger than a conventional gas engine of similar displacement.