It seems that the paper is making theoretical strides in the limitations of producing a positron beam via the Breit-Wheeler process. From Wikipedia

This mechanism is theoretically characterized by a very weak probability, so producing a significant number of pairs requires two extremely bright, collimated sources of photons having photon energy close to or above the electron and positron rest mass energy. Manufacturing such a source, for instance, a gamma-ray laser, is still a technological challenge.

I’m not claiming I know this stuff. I was just trying to figure out what was new here since I didn’t find the headline very surprising.