For the first time ever I saw a comparison between a good air cooler and an AIO running on a dual chip Ryzen CPU and as I suspected see Definitive guide to configuring the Ryzen 3900X , the air cooler outperforms the AIO.
This is because the air cooler has 100% of its cooling capacity implemented, that is covering the hotspots of the two chiplets; whereas an AIO only has at best 50% of its cooling capacity (where the heat exchanging fins are in the cooler head) covering those hotspots.
Source: Hardware Cannucks
The Noctua NH-U12S is not even the best 120mm cooler that Noctua has to offer, that would be the Noctua NH-U12A.
A Noctua NH-U12A would knock another 6-7°C off the two fan temp of the NH-U12S.
So I would take the recommendation of AMD to use at least a 280mm AIO to cool the 3950X with a HUGE grain of salt.