What he is saying is that the car will heat the battery to optimize the temperature of the pack. It will do this to enable higher power output and to prevent degradation at the cost of range. It uses a heater to speed the process. Would it get there on it's own, yes. Would it incur additional degradation to the cells/pack? Tesla seems to think so. Also remember that the drive unit is very efficient so it would take quite some time to get the pack up to temp from a cold soak in sub-freezing temps.
At a Supercharger it's usually the opposite, even in the winter. The charging rate alone is enough to keep the battery plenty warm. Too warm is also a big no no.
Edit: Looks like charging while cold is really the only thing that will actually degrade the cells. But the amount of power output and capacity is limited when cold.
Discharging at High and Low Temperatures