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Average power density is power over die area, but hotspots concentrate several times that in small regions. Since thermal resistance is local, it is the hotspot density that determines peak junction temperature and therefore whether the part throttles.
Power density
average = P / A; hotspot = average x concentration ratio
Heat spreads only a few millimetres laterally in silicon, so a small high-power block can be tens of degrees hotter than the die average.
Around 0.5-1 W/mm2 with a good heatsink, several W/mm2 with direct liquid cooling. Above that requires microfluidics or aggressive throttling.