Does kale taste sweeter and more flavorful after being frozen?
You have misread the Wikipedia quote. It says (emphasis added):


  Kale freezes well and actually tastes sweeter and more flavourful after being exposed to a frost. 


While not terribly well written (since there is the red herring that it freezes well), this doesn't mean that the harvested plant was frozen, but rather that the live plant in the ground was exposed to frost.  This causes it to begin converting starches to sugars in preparation for winter, giving it a sweeter taste.

Per Burpee, for example:


  Frost enhances the flavor. Some of the tastiest kale is harvested
  under a foot of snow! Never harvest kale until after a hard frost or
  two. A few freezing nights make all the difference in flavor as the
  kale plants need a hard frost to transform their starches into natural
  sugars.