The authors of the study are claiming it as an emission line, but not all emission lines come from transitions of electrons. Some come from changes in quantum rotation states (e.g., some molecular transitions), rotation-vibration states (other molecular transitions), or flips in the spin direction of electrons (the famous hydrogen 21cm line). In this case, the authors have not found known lines from molecules or atoms which would correspond to this energy.
The suggestion of dark energy comes from some theories where postulated dark matter particles can annihilate if they hit another dark matter particle. In these theories, the result is emission of high energy photons with an energy which is related to the mass of the individual dark matter particle. Basically E=mc^2. So it is still an emission line, but not one that arises from an election transition in an atom or molecule.
The suggestion of dark energy comes from some theories where postulated dark matter particles can annihilate if they hit another dark matter particle. In these theories, the result is emission of high energy photons with an energy which is related to the mass of the individual dark matter particle. Basically E=mc^2. So it is still an emission line, but not one that arises from an election transition in an atom or molecule.