It's a render. A real life takeoff video is still missing..

Movie S5 from the supplementary material: https://imgur.com/a/LVb2m22

You can see the string attached to it! Do they think we're stupid??

The string goes slack when it lifts off, so it's not like it's being pulled up. I think it's there to catch it in the inevitable case of a crash. And maybe they haven't figured out how to land gracefully yet.

Looks super cool. Now I want to build one and stick an FPV camera on it:)