The preprint of the paper that is the subject of the (not so great) phys.org article at the top is <https://arxiv.org/abs/2205.08767>. An accessible HTML5 version is available at <https://ar5iv.org/abs/2205.08767> (arxiv->ar5iv, the latter expands to a link within ar5iv.labs.arxiv.org).

Hawking radiation is a semiclassical result: the curved spacetime is classical General Relativity and the scalar field (in which Hawking quanta arise near the central black hole) is quantum.

The dynamical spacetime creates -- through the equivalence principle -- an acceleration between past observers and future observers, and this acceleration corresponds with the Unruh effect. The Unruh effect rests on the definition of a vacuum as a state in which an observer sees no particles, and that when an observer accelerates a no particle state may be transformed into a state with particles. Equivalently, differently-accelerated observers will count different numbers of particles in a spacetime-filling quantum field. (A family of observers may count no particles, i.e., it's vacuum.)

The important part here is that a dynamical spacetime ("gravity") and a relativistic quantum field is needed for Hawking radiation.

So, "[can] spacetime alone ... create light?" No. There must be a matter field filling the spacetime. That matter field, if quantum, can look like it has no particles in it to some observers, but not all observers. The dynamical evolution of the spacetime can cause observers' counts of particles to evolve.

> gravitational waves ... carry energy, so can be transformed into light

The paper is about how, given:

* a massless quantum field theory proxying for light

* a quantum field theory in which gravitation is mediated by a massless spin-2 boson

* a dense medium with a (light-) refractive index greater than 1

* standing gravitational waves of significant amplitude occur in cases where gravitational radiation from widely separated sources converge within the dense medium and somehow [a] cancel out polariation and [b] are within a wide (compared to the wavelength) patch of flat spacetime

* the non-light massive and massless particles within the medium couple very weakly to the incoming gravitational radiation

* the particles of the refracting medium couple weakly to the "light" field, and generate practically no spacetime curvature even in bulk

then the light-proxying particles may be produced via a process which the authors compare with electron-positron pair production and Cherenkov radiation. (Although they do the latter comparison very very breezily, not delving into the cross section of light-by-light scattering).

There are weaknesses in this list of requirements, some of which the authors admit requires further study.

The key point though is that their mechanism cannot work in vacuum.
It absolutely requires that the light travels significantly slower than the gravitational radiation (which in turn is assumed to travel at c, even in the non-vacuum in which light travels slower than that) and that a far-from-negligible momentum is lost by the incoming gravitational radiation as it passes through the refracting medium.

> great experiment here

The last paragraph in the Conclusions and Discussion section suggests there may be avenues for experimenting with the ideas in the paper.