Because a solder layer would have much higher thermal transfer rate per area than thermal paste. And the IHS is bigger than the die, resulting in better cooling (because there is more 'bandwidth' (=area) at the point where TIM is used).. which was pretty much explained in the article.

The heat sink could easily replace the role of the heat spreader if it was soldered on. It's much bigger. I could definitely see processors moving to an integrated and standardized heat sink.

That doesn't make sense - TIM is used to fill the gaps where the copper and silicon don't have direct contact (that's why you use as little of it as possible), not as a heatspreader itself. A direct heatsink-silicon contact will almost always be better for thermal conductivity...