That means they are not "demo" vehicles, but were titled to some end user consumer before. A car can have miles and be sold as "NEW" provided it was never titled to anyone. Demo cars generally are not titled to anyone, so are sold as NEW.
What both of those cars likely are, are cars that were sold to someone, then returned for some reason, THEN turned into demo cars for the dealership.
So, if the car has extremely low miles like that, but is being sold as used, you can guarantee that it was titled to someone (meaning sold to someone) then returned.
Exactly.
I think the OP should contact the SA that told him this and verify that the cars have actually been titled previously, as SA's are well known for spreading misinformation (to their credit, not intentionally).
So rather than ask the same question again ("are you sure that the cars you're talking about will be sold as "used" cars?"), ask him/her if the cars in question have ever been titled to anyone since the time they were built.
I'd be willing to bet that the answer is "no, they haven't ever been titled." Tesla did away with their 7 day return program long enough ago that any remaining cars turned in under that program should be long gone by now.
Also, Tesla always provides an Odometer Statement showing that their new cars have 50 miles on them. This is a bit disingenuous on their part, but it does make sure that it covers any odd instance of a new car having a few more than the standard amount of miles on them when they're delivered, while eliminating the hassle of actually finding out the true odometer reading on each car (pretty weak sauce on this one, Tesla).
From what I've read, they also have a blanket policy of stating that vehicles used for test drives are always stated to have 500 miles on the odometer when sold, even though they are usually sold before they ever reach that point. So just because a vehicle is shown in their inventory as having 500 miles on it actually means that the car has somewhere between 0 and 500 miles on them. I can totally see a new and/or uneducated Sales Advisor assuming that cars shown in inventory that have 500 miles on the clock are "used" cars, even though they've never actually been titled to anyone.
If it turns out that the cars really have been titled before, then go back and re-read JJ's post. Carefully.

IMO, before purchasing any car from Tesla that has very low miles on it (say.... 500

) and has been titled before, you should do a very thorough pre-purchase inspection of the vehicle; don't go buying one of these cars sight unseen.
Arming yourself with those little tidbits might help you make a better decision and understand a bit better what the status of an inventory car is before pulling the trigger.