...and I eventually bought both ASRock Z390 Phantom Gaming and Noctua L12S. It fits !
I mounted it horizontally, with the strips parallel to the RAM, the heat pipes touching the i/o heatsink. As said for the L12, contact between i/o shield and the heat pipes makes the L12S slightly bend, making the fan applying pressure on the RAM at the other side (not that much, still very acceptable).
Anyway it fits the Nouvolo Steck (announced with 70mm CPU heatsink clearance).
The picture below was for another purpose but you can see the L12S is not perfectly parallel to the side panel, because of the slight bend caused by the io shield/heatsink on the left. You can also see how it sits on the RAM (great cooling for them !).