Hi there,
Although I didn't read that mag (it is kind of hard to get it over here), the basic thing about the way the ACD works explains why it might not be as sensible at the track at the gravel or snow settings. Here is the thing.
When the center diff is in the full lock mode (50/50), it creates a lot of push on both sides of the car. Car basically goes wider than it should, since both ends are pushing, but it is very stable and it is hard to spin the car.
With a center diff fully open, power goes wherever is easier for it to go (like regular open diff). In the case of the full throttle application, when car is entering the turn, most of the power will go to the rear end and spin is most likely going to happen. The fact that the most of the weight is in the front anyway, helps as well.
So, what they try to do with the ACD is to control how much of the oversteer the car is going to need. In other words, the question is how much (or how long) the center diff will be in the open mode (which will induce oversteer). I am purposely ignoring the fact that there is also AYC in there and it helps a lot, but just for the simplicity, it is not mentioned.
Now, when comparing traction between the dry tarmac vs. gravel or snow, it is obvious that slippery surface require less open diff than the more gripy one. So, car will have certain characteristics in the tarmac mode (and this is usually the most aggressive oversteer mode) and it will allow drivers to rotate the rear end let say quite a lot. Now by switching to the more slippery settings, center diff will stay more in the locked mode and in the case of the dry tarmac, it will be more difficult to rotate the rear end.
I hope this was helpful!
Fedja