a cooling unit provides a delta between surface area temparature and the ambient temparature. Epyc chips are significantly larger and more spread out with the chiplets so the heat density of these chips are relatively similar. So the cooler doesn't have to provide a larger temparature delta and ony has to be slightly larger if at all.

Overall heat density on chips has increased due to litography changes so the chiplet architecture in a way is only a stopgap.