Not sure about any source tbh not really sure how the plumbing works out with bandwith perhaps sata ports and sata express can also eat into that bandwith?
Otherwise why is the M2 slot making sata ports not usable ?
But I do agree with your maths on 5820k and say SLI/Crossfire and 16x/8x config and still having a 4x (out of 28 lanes) being available, only reason why I can think that motherboard manufacturers are adding that disclaimer in the manual is something else may be using up the bandwith just like a sata drive or sata express drive.
It hurts my head even considering it ! which is why I like the 5930k with 40 lanes.