Is many small propellers really more efficient than few big ones? Even if they didn't want to make a mini-helicopter, they could've also went with a 3-4 rotor design.
That being said, this is only aero efficiency. Once you start to add electric motor efficiency, things get interesting. To generate thrust with lower number of blades you need to have large torque torque to put into the prop , which means the motor has to be a low Kv (Kv = rpm/volt, and a low Kv means a high Kt where Kt = torque/amp), to keep the current lower to lessen electrical power losses. Or use a reduction gearbox. Both of the above necessitate a bigger package and more weight, which requires more thrust, and so on.
There is a place in this design space where having multiple rotors is actually more efficient for the overall flight envelope.
Smallish helicopters aren't that expensive. Maintenance and fuel are though. Going electric already helps with the fuel, going for many motors helps with the maintenance.
The range of an electric helicopter is going to make it impractical, and I doubt that's going to change any time soon.
Gas stores energy in 100x less weight than lithium ion. Electric motors are 2x more efficient.
You're still going to need space and weight for ~50x more battery, which is hard to come by...