A demo can be a new car, so those terms are not exclusive to one another, and "demo" isnt a regulated term. Its up to you to ask Tesla that. That amount of miles is more than a typical "brand new never driven" car would have, so it moved around for some reason. It could be as simple as they drove it from one location to another.
New cars that havent been driven except for build and delivery tasks usually have less than 15 miles on them.
With that being said, I will say one more time a car being a "demo" or not has zero to do with whether its being sold as new or used. A car that was used by a dealership and driven for 1000 US miles can still be sold as a new car, depending on whether they took ownership of it or not.
Your question is more along the line of "was this car used previously, or what is the reason for these additional miles?" and you need to ask tesla about that.
