They're looking for and describing proximate relationships (see e.g. https://en.wikipedia.org/wiki/Proximate_and_ultimate_causation).
If the light isn't warm and insects still move towards it, that means they're not using heat to make the decision where to move. They're not saying anything about the ultimate underlying motives, just the immediate proximate mechanism.
Their alternative model is that it's based on orientation to the sky, by which insects point the top of their body (dorsal surface) toward light; if the sky is bright that would keep them oriented back-upward, but if it's a point source near the ground keeping their back towards the light causes them to fly in circles around the light:
Our guidance model demonstrates that this dorsal tilting is sufficient to create the seemingly erratic flight paths of insects near lights and is the most plausible model for why flying insects gather at artificial lights.
So, if you want to propose heat-seeking as a better hypothesis, you'd need to provide data that would explain why this dorsal facing tilt would make sense for an insect seeking heat.