According to this wikipedia article it does.
However a chef at my restaurant had not heard of this.
The wikipedia article seems to be missing citations, and I don't have time to perform an experiment.
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.
I found this online. Doesn't say anything about freezing, but frost does it too. So, I would think frost makes it sweeter.