I think aab0 means that this particular galaxy is ruled out as a Dyson sphere, not the possibility of there being one somewhere in the universe. A Dyson sphere

I think aab0 means that this particular galaxy is ruled out as a Dyson sphere, not the possibility of there being one somewhere in the universe. A Dyson sphere should still emit infrared, unless the aliens can subvert thermodynamics, at which point anything is possible.