I think the more rigorous way to get at the OPs point is to observe that the AUC is the probability that a randomly selected positive class is scored higher (by your fixed model) than a randomly chosen negative class. Being probabilities, these are independent (at a population level) of the number of samples you have from your positive and negative populations (of course, smaller samples get you more sampling variance). I believe this is the OPs point with "they are fractions".

In any case, can we at least all agree that blogs/articles throwing around this kind of advice without justification is less than helpful?