Another data point: One of the main reasons I ended up buying an inventory car was because it included free supercharging. I had test driven an AP1 car in Dec 2016, and wanted to wait for AP2. My car was built in late Nov, 2016. Order was placed and deposit put down in early Feb 2017. I picked it up from Tesla the last day of March 2017. It has free supercharging. The current text in my Tesla account (which has changed several times since I bought the car...):
So, in actuality, for the non-referal based "free supercharging" (the kind that follows the car), Tesla seems to be using the car's build date (and possibly also the customer's order date, whichever is earlier) to determine eligibility. If your car was built before Jan 31, 2018 or you placed your order before Jan 31, 2018, you are likely eligible.