I have no idea how true this is, but the press releases claimed that both most of the capacity is retained down to -40 and that the charging speed is proportionally retain down to -40, and that this is the meaning of the operational range.

When you're at a charger you can easily spare the 3-4% of battery capacity it takes to heat the pack by 40C. Less if you use a heat pump.

How many people let their ICE engine block get that cold?

i would have imagined that charging at -40 is easier than operating at -40.