I think he's saying the 2.8 point is more precise than the 5.6 points not that they won't work. And I believe reading the manual on my 5d3 the center point is more precise for 2.8 and faster lenses. I'm not a expert at this topic but that's how I understand it. Not that it won't work but that it's more precise. How much more precise IDK
There's a spec on the accuracy of a focus point. It used to be 1/3 of the DOF. So, if a point is good to 1/3 of the DOF at f/5.6, it should be good to 1 DOF at f/1.8.
Now, we don't know if that's 1 standard deviation or more, and we don't know if the spec on the 7D2's focus points are 1/3 of the DOF or something else. That's why I wanted evidence. I suspect that the spec is tighter now than it was when 1/3 of the DOF was the standard but I don't know that either.
With 1/3 DOF f/5.6 points I get around 80% in-focus images at f/1.4. If the 7D2 can do that well, I'll be happy. Since that spec is 10 years old, I suspect it will do better and that the reason it only has one f/2.8 point is that it doesn't really need the high precision points to get a good percentage of in-focus images with f/2.8 lenses like the 300/2.8 and 400/2.8. It's also probable that having multiple focus points all working on one subject will further tighten the performance since errors will tend to average out. And with all of the 7D2's focus points being cross points, you have at least two on every subject.
This is why I want to see some testing on faster lenses using other than the center point.