Yes, in the conclusions of that paper:
"Observations of a single flare cannot distinguish the quantum-gravity scenarios considered here from modified synchrotron self-Compton mechanisms [23, 31]."
 is Bendarek and Wagner, published the same year (2008). Both of those authors are on both papers.
They need more observations, as they say, to distinguish between Quantum gravity effects and plasma effects.
I don't think anybody doubts that the universe is expanding. What I think needs further proof is that it is accelerating.