The cooler itself probably isn't that much worse than the Nvidia ones in terms of cooling power, it's just that clearly the 290x core is less efficient than the 780 and Titan cores which is evident from the power consumption (and exaggerated by the throttling on Nvidia cards). The 290x cooler inevitably has to work harder as it has more heat to dissipate.
This is what DM is saying, and he's right. Without bolting a 780s cooler on a 290x, or vice-versa, there's no way of knowing for sure. Saying 'it looks rubbish so it must be' is idiotic, as is assuming that the Nvidia ones are amazeballs because they look well made and the cards run a little cooler.