Without being aircraft expert the KH design looks trustable.

How do they manage to make it so quiet? Is it a feature of the propeller blades, such as material used or angle of attack, or perhaps a lower number of revolutions but spanning many engines that does it?

When it's flying along, the props don't need to be spinning as fast. When it's doing vertical take off and landing, Heaviside (weird name) is making as much noise as all those 8 rotors can make to keep it hovering. It's an improvement.

One big air beater beat has greater volume low pitch penetration than many small out of sound synch beats beating at higher pitch. The future will be no air beaters using just elastic charge propulsion.

Imagine being able to have an airport anywhere within a city or suburb because the planes are so quiet you don't even know there there. And pollution free. This is the future of flight and for all flights under 1,000 miles they will be electric. Slower yes but quiet and pollution free. And far safer. If they crash they won't burst into flames making survivorship more possible.

Somebody better wake up Limbaugh who, today, vehemently insisted all-electric aircraft large enough to ferry humans was a physical impossibility.

Yes interesting progress when coupled with the battery advances (kg/kWh) that are in development. Now one problem with upscaling is noise: biggest part of big rotor or propellers is their tips running at above sonic speeds. Another R&D topic which needs to be resolved.

Is it just me or did that look like a model when it landed at the end? It seemed small the way it just "plopped" onto the ground. That would be a pretty heavy landing for a full size aircraft.

Total bollocks of a comparison, a 6kg model compared to a helicopter for noise output...
At the size of this craft, anything can fly for 10 or 20 minutes.
I love and work with VTOL and fixed wing aircraft... Any professional in the field has to imagine the impossible and implausible while living in the tedious present.
The general public OTOH laps it all up.

If they can do the 38dB on vertical, they'll sell a million of them quickly. NOT adding to noise is a key point in cities. Best of luck, Heaviside!