If a "simple" innovation like that can move the needle juste by a few percent points on how much fuel we put into the transport industry, that's a very sizeable gain from a climate perspective.
Let's imagine 3% of improved efficiency (they announce between 9 and 15%, but let's be conservative) on the 3% of the world's emission, that's still almost 0.1% of the world's emission saved by only changing the propellers on cargo ships. It's still a massive improvement: 31 million tons of CO2 saved. At this scale, anything that moves the needle a little bit should be taken. But that's by plucking a number out of thin air, so I'd love to hear about it in the real world and at scale.
Regarding the rest of your message, I agree that the type of fuel that is burned is a big problem, but if changing a propeller means burning x% less of it, it's a no brainer to me!
Haven't checked the maths but energy use in buildings is a far bigger chunk of emissions than shipping.