The Earth's atmosphere absorbs and reflects solar energy - only 48% of the solar energy reaches the surface of the planet[1]. I didn't see any mention of this on the link you provided, I don't know if that 6-9% figure includes this.

If it's not included, then the extra energy available could raise the output to an equivalent of a 12-18% efficiency cell on Earth's surface, wouldn't it?

[1] https://earthobservatory.nasa.gov/features/EnergyBalance/pag...