I was surprised because I thought the Breit-Wheeler Effect has been demonstrated already. It just looks like what they mean here is single photon (or really, two photon), not the so-called strong field Breit-Wheeler that was demonstrated at SLAC and with powerful lasers for decades now.
One of the authors here. You are correct, the famous SLAC result already demonstrated that light can be converted into matter, but they needed several photons >4, average of about 6 - that is the so called non-linear Breit-Wheeler (BW) process. This result is the original linear BW process where only 2 photons collide. The main difference is that for the linear BW you need much higher energy photons. Also, in this experiment we are able to measure the wavefunction of the two-photon system, which proves other aspects of the original predictions by Breit & Wheeler and others.
I was always curious before I heard of this today what would happen if you concentrated more and more light into a ultra fine point. It makes sense that at some point particles would begin to emerge as photons appear to have various electromagnetic properties and also carry a small amount of kinetic force (solar sail principle)