It means that the conversation isn't producing much of value despite lots of activity, like an inefficient lightbulb that wastes energy by emitting heat instead of light.

It's an idiom implying that there's a lot of chatter and bold claims, but very little of it is factual or informative.

Incandescent light bulbs are generally very inefficient in producing light, compared to LED for example. They produce a lot of heat and not much light for which they are made.

So in this context I suppose that gp implies that these threads don't provide much meaningful discussion but rather lots of hand waving.

Emotion-to-understanding ratio

Heat = Flaming, Light = Illumination (in the metaphorical sense)

It’s trying to say light is more valuable than heat, or some such folksy thing. I cook steak in the dark so I don’t find it to be a very insightful metaphor.