I thought all MY's have been HW4 for quite some time now... at least since May. If you're really concerned, you can ask the SA about a specific VIN.
They're giving deep discounts to drive up end-of-the-year numbers. Dealerships do the same thing, its just less visible because they do it through rebates and stuff like that. Plus if they get a customer willing to pay more, they'll gladly take their money. Tesla has to be more upfront due to their business model and standardized pricing. They're really looking to clear out Model 3's to make room for the refresh. I imagine you'll see even deeper discounts on MY's when that refresh is nearer as well.