From the article.
"Using the Nissan Leaf electric car (here) as an example, Dr Collett said: “Say the heating was on constantly at 2kW (which would be unlikely due to insulation of the vehicle), even if you only had half the battery charged, you would still have almost 10 hours of heating."
They must have been talking about a half-empty 40kw Leaf?
Lets say you have a 62kw Leaf and it was down to say 50kw, using their 2kw figure one would have just over 24hrs and again unless the temps were below zero I'd guess more like 1kw would keep the Leaf acceptably warm. I mean think about a 1kw heater in a room the size of the Leaf cabin, I think that would be doable.