> but I guess you get into problems with it being ice and snow that you're testing
If it's not actively snowing for a few days roads get clear by thawing during day because of salt/cars being hot/sun shining.
The "summer tyres get too hard in the cold" idea could very well come from places where the typical winter is at least < -5°C. It's not uncommon in central/eastern/northern europe for temperatures to be < -10°C for extended periods of time.
The linked article claims "in dry conditions, summer tires have the best grip down to 0C", though, which doesn't seem to match where you suggest the crossover point is.
winter tires, especially the snow versions, are straight out a source of danger here in central europe.
i just put some snow chains in for the winter surptises, and ride with my summer tires because its grip is clearly better on dry and wet bithumen, even in near zero temps, which is most of the season.
If it's not actively snowing for a few days roads get clear by thawing during day because of salt/cars being hot/sun shining.
The "summer tyres get too hard in the cold" idea could very well come from places where the typical winter is at least < -5°C. It's not uncommon in central/eastern/northern europe for temperatures to be < -10°C for extended periods of time.