Straight from the horse's mouth:

"1000 kilometers... hmm. Well, it depends under what circumstances for a thousand kilometers. As it is, the record right now for model s is 800 kilometers (496 miles). That's the furthest that anyone has driven a model s. ... Now, in order to do that, they did drive at a relatively slow speed, so, you know, we're talking, I think they drive at 40 or 50 kilometers an hour (25-30mph) or something like that. But I think, my guess, we could probably break a thousand kilometers within a year or two. ... 5 to 10% a year"

It seems, to me, slow driving ("circumstances") is what he had in mind.