So could any other type of antenna, basically. It doesn't have to be a dipole, though.
> or in the center of a parabolic reflector
Based on the images, that's not what happens here
> There is no possible way to increase on efficiency in a passive antenna.
That is absolute horseshit. Large dipole arrays will suffer from high mutual coupling which could drastically reduce performance (depending on design choices). Different antenna types might be more suitable for different scenarios, but that's why I would like EmRod to explain their choices.
Furthermore, a rectenna gives you DC power out, but why would EmRod want this?
A Rectenna is just a dipole with a rectifier at the feed point.
They could be in a phased array (of dipoles), or in the center of a parabolic reflector.
There is no possible way to increase on efficiency in a passive antenna.