The article is light on details, but there's details elsewhere.
The OPOC engine is a horizontally opposed two cylinder two-stroke engine. As a cylinder in a two-stroke engine has a power stroke on every revolution instead of every second revolution, this engine has very high power density compared to a four-stroke engine of the same size.
Traditionally, two-stroke engines have had very poor emissions. Since the exhaust and intake strokes are not separate, the intake mixes with the exhaust to some degree. This means that some of the intake fuel goes out the exhaust unburned, and some of the exhaust remains in the cylinder with the intake charge, reducing peak temperature. This engine, however, uses assisted HCCI with a diesel injection system, meaning the fuel is introduced during compression instead of intake, so unburned intake fuel does not cross over to the exhaust. (I'm not clear what the "assisted" part is in the assisted HCCI. Perhaps there's a spark plug that's only used during low-power, lean burn conditions?)
The cylinder pairs are intended to be balanced and stackable, so that multiples can be connected together for higher output. TFA suggests that it might even be stacked with an electric motor for low-speed operation.
I imagine these would be very useful for a hybrid, despite the summary title. Unassisted HCCI engines have a small power range, but this would be perfectly fine for a series hybrid generator motor running at a fixed RPM for charging.