It was hard to see, but it looked like handwriting to me.

For marketing reasons, it needs to be something that people can pick up with absolutely minimal practice.

I doubt it has enough accuracy for a virtual keyboards (since keyboards require precise absolute input and it measures relative), besides, most people aren't experienced with single-hand typing.

A bespoke gesture based shorthand would be optimal, but then users would need to spend months learning this new shorthand.

But (almost) everyone already has experience with handwriting, which is a single hand relative input method. It's the easiest option for people to quickly pick up and enjoy.

Though, it's far from perfect, you can see he is struggling to trick his muscle memory into writing without a pen, and he needs to do it on a solid surface (I'm not sure if that's a technology limitation, or a muscle memory limitation).