I wouldn't necessarily say that. Go read about the lengths D800E users have to go to in order to get all the resolution out of their sensors. You have to use the _very_ best glass and hit the perfect f/stop with the perfect focusing (hyperfocal generally). It is already getting quite fiddly to feed 36mp... if Canon is going to do 46mp things are definitely going to get interesting.
I don't consider a 7D that difficult and it has a higher pixel pitch than the D800. There are plenty of lenses that can provide 46 MP of detail FF (18 MP crop).
Diffraction is not the hard, fast limit people think it is. There's a gradual loss of detail contrast. That's something people don't understand when discussing diffraction or lens testing. They will, for example, read that a lens "only" provides XY lpmm at 50% MTF, or that you can "only" achieve XY lpmm at Z aperture, and assume no sensor with smaller pixels can benefit. What they don't realize is that the lens or aperture setting can still saturate any sensor at 20% MTF. And in our digital world, PS can bring back much of the lost detail contrast.
You run into similar nonsense comparing FF and APS-C sensors, and especially comparing lens resolution on the two formats because of the way the major sites compute the resolution numbers.