Indeed - there is enough aluminum/copper in the SoC's metal lid and PCB's inner (ground/power) layers, that the whole thing is effectively a 2-3 square-inch heat-sink. Eben himself has been talking about this technique around the time RPi3+ was released.

And your point here is completely understandable too - there is a kinda-heatsink there already. Just add a fan, and we're good.

My point is - seeing a large black hole where SoC goes (color corresponding to +20-ish degrees C by the chart) is utterly confusing, since SoC is the hottest point actually.