Not sure if the author was asking a question that he subsequently answered (I haven't read this article in a long while), but the facts are that he was never able to find condensation to have been the reason for water accumulation.
It has to do with the difference in MASS. The liquid is more temperature stable than the air and the containment vessel. I would state that it's more about the existing air in the containment vessel changing temperatures than it is about warmer air entering the containment vessel: air will enter when fuel is puled out, I'll give that, but the rate of change is pretty insignificant. The fuel will tend to be warmer and the air will, through conduction, be cooled down, with the coldest points being along the containment vessel (it's at this juncture that condensation forms).
The author gives numbers on how much moisture that the air can contain and shows how insignificant. But, yes, over a long period of time there can be a meaningful amount of water accumulation: and that amount of time seems to be a lot longer than most would figure.
I picked up an old diesel generator that had perhaps 1/2 tank of fuel (6 gallons; tank size 12 gallons) that was something like 5 years old or such. Generator had been sitting for all that time. Again, NO water.
