100% duty cycle doesn't really exists as stand alone concept. It is always in reference to the application. A typical car engine is at 100% duty cycle at around 30 horsepower, but the engine can produce 200+ horsepower for short bursts (enough to do the 0-60 test that everyone loves to quote). With more cooling you can run more power from the same engine, but the additional stress will mean the engine doesn't last as long before it breaks.
Marine applications generally means salt water, so they will run at higher power is the salt is what gets the engine eventually not the stress of high power output. Airplane engines are expected to be reliable and so they run at relatively lower power output. There are many small niches in between that have their own idea of what 100% duty cycle is.
I agree, it can mean different things. To be more specific, the video I linked says Kia tests at full load, WOT, and maximum engine speed for 300 hours continuously. Then they run it 10-20% faster than the specified maximum engine speed for another 10-20 hours.
This exceeds most definitions of "100% duty cycle".
Marine applications generally means salt water, so they will run at higher power is the salt is what gets the engine eventually not the stress of high power output. Airplane engines are expected to be reliable and so they run at relatively lower power output. There are many small niches in between that have their own idea of what 100% duty cycle is.