An interesting article, but I'd like to make a couple of points:
1. I don't think the extremes of temperature given for a guppy necessarily represent the daily temperature variation - it may refer to seasonal variation, or regional variation;
2. The 'bathythermograph' study is interesting, but of limited relevance, since most aquarium species do not come from 2000m-deep watercourses; it doesn't tell us about temperature changes over time.
3. I would like to see some evidence that water temperature equalises with air temperature in nature, given the insulatory properties of the earth, and of the water itself (note, for example, underground houses can have an almost constant temperature all year round). On the other hand, temperature in an aquarium would certainly equalise with air temp. pretty quickly, being a relatively tiny volume of water with little insulation.
4. While a guppy may tolerate a wide temperature range, a species that evolved in a more stable environment may be less able to adapt.
I like the basic thrust of the argument, but I wanted to raise those points. I don't have air conditioning, and in summer my tank temperature is regularly fluctuating at least 5 degrees C, with seemingly little ill effect (in fact it seems to encourage breeding) except on the odd extremely hot day when some fish look a bit dopey (oxygen-deprived?)
I hope the discussion throws up some more info, as I am quite interested in the question of temp. fluctuation and fish health.