It's both. This design focuses on quieting the airflow. From TFA:

The team started working last September. They hoped to improve an electric or battery-powered leaf blower, which is already much quieter than the notorious gas-powered ones, where the sound can carry over an average suburban block.

No. Nearly all of it is the engine.

I don't believe that part - the ones shown in the picture/videos are brushless motors, quite efficient (90%) and really silent on their right own.

The noise comes from the need to move air (and the noise is just air waves), along with certain vibrations. A brushless motor is super silent if you exclude the fan(s) cooling it.

Electric doesn't have engine. It has a motor. Gas has an engine.

https://engineering.mit.edu/engage/ask-an-engineer/whats-the... by the way...

Which is why I said engine, not motor.

The topic is an electric leaf blower though. While a gas leaf blower is louder the difference isn't as most of the noise is the blower.