It's still a decent improvement if they are - one big coal plant running at relatively constant speed is much more efficient than an equivalent number of individual petrol engines in cars afaik.
Coal is one thing, the environmental cost of producing batteries is another. You have to drive your 100kWh Tesla for over 8 years to get carbon emissions lower than ICE cars[1].
That number probably got lower thanks to Gigafactory being more efficient. Hopefully.