I think the problem is that, as written, there is nothing to read into the statement at all, because it does not say anything.
Adders have to be based on an increase of the raceway temperature (and therefore the even higher conductor temperature) above what they would otherwise be.
If all you do is measure overall ambient temperatures and average them, you have learned nothing except what the average ambient temperature is.
If the ambient temperatures they are averaging are the
actual raceway temperatures, then you have some data.
And the natural way to get the adders is to take 10 different ambient temperatures and the corresponding circular raceway temperatures, and average both sets of numbers and then take the difference or else take the 10 differences and then average them (which will always give you exactly the same number!).
That could be described as averaging the adders corresponding to each ambient temperature.
There is nothing there that tells me that the adders should only be used to correct the
average ambient temperature. (Although in all fairness, it does not tell me to do anything else either.) It is implicit in "adders" that I have to add them to something, and in the absence of any direction to the contrary I would add them to whatever temperature I would have used had I not been told to apply the correction.