It's very difficult to compare total different AF systems with each other. The Canon AF system of the 5D3 and the 1Dx is even now, much more advanced then the Nikon D4s or the D810. What Northrup want to tell us that the D810 has a higher keeper rate compared to the 5D3 is his opinion, but I'm sure he is not really aware then of all the possibilities of the much more advanced AF of the 5D3.
It's your opinion
that the AF system of the 5D3 is more advanced than what the Nikon D4s & D810 offer.
It's my opinion that Nikon's is far more more advanced in practice
. Simply b/c of the 91k pixel RGB metering sensor for subject tracking.
Our difference in opinion probably boils down to you not caring about iTR on Canon, or Nikon's analog: 3D tracking. (considering you said: "So even the iTr on a 1Dx is hardly used in my case because of the lower AF speeds of the zone and 61 points AF compared to the other methods.")
Nikon's 3D AF tracking enables me to capture shots I wouldn't have dreamt of catching with my 5D3. Using fast f/1.4 wider primes (24mm and 35mm), I can always use the center point to initiate tracking, and then keep that subject in focus even after recomposition or the subject moving (which babies & running brides tend to do a lot). My hit rate for this type of photography shot up dramatically going from a 5D3 to a D810. And I wonder how many people actually realize that 3D AF tracking can be used for this to such great benefit. The 91k-pixel sensor does remarkably well in this regard of recognizing, say, an eye & sticking with it - it's so good sometimes, and so fast at shifting the AF point, that sometimes it feels like the camera is using the accelerometer data to measure my hand movements in order to shift the AF point (obviously, this isn't actually the case)!
It's funny - the term '3D' here is almost misleading, as it implies depth. Well, AF in general (in AF-C or Servo modes) tracks depth by default. The point of '3D' here is that it also tracks across the two dimensions of the frame; hence 2D + depth = 3D. But, at first, it seems counterintuitive, if you see what I mean - it's the 3D mode that tracks across the 2D plane, and all the other modes that automatically track depth (in AF-C/Servo).
Now, there are some other advanced features Canon offers - 5 high sensitivity dual cross-type points in the center with wider baselines, more cross-type points on the sides, and spot AF. Of these, I miss the cross-type points on the sides the most. I have yet to see benefits of the high sensitivity points, and spot AF, for my work. I'd love to quantify how useful these can be, especially the high sensitive wider baseline points for low light work. Roger Cicala's work showed no real difference in precision between the modern Nikon and Canon AF systems, but I believe that was in good light.
The 1Dx offered *the potential* to combine all those pluses of the Canon AF system with the '3D' tracking capability Nikon's offered for a long time (and Canon did as well, just using only depth information from the AF system - in other words, it didn't do it very well). Unfortunately, I never found it able to keep up, and stick as well to the initial subject, as Nikon's D810 or D4s. No matter what combination of use-cases/settings within those use-cases I tried on the 1Dx. The 1Dx is, however, much more capable than the 5D3 at this type of AF. It approaches the Nikons in this regard, but still has some catching up to do - as you'd expect for a 1st generation tech.
So, for my type of shooting, the AF system on the D810 far outperforms the 5D3 I used extensively for 3 years. And that was like icing on the cake, since I'd been wanting a better sensor for a long time.
YMMV for the type of photography you do, of course. If 3D AF tracking doesn't matter to you, the 5D3/1Dx offer very compelling AF systems.
It's just that when you make blanket systems like 'the much more advanced AF system of the 5D3', I really can't sit silent...