I agree it can be hard to confirm if a 2017 has transferrable supercharging, but not because the requirements are murky.
The original order cut-off was end of 2016. That where hearing it had to be order by Jan 1st might originate. However, what that fails to recognize is that Tesla at the last minute extended the order cut-off to Jan 15, 2017. Reason for the last minute extension is that Tesla's website couldn't deal with the volume of traffic from people trying to order at the last minute. The result was Tesla extended the order cut-off point in a move of trying to acknowledge issue on their side.
Now where it gets murky is there's no clean way for anyone to know from VIN or build date whether the car was ordered by Jan 17, 2017. That's where you can have two cars, built on the same day in 1Q 2017, delivered on the same day in 1Q 2017, but one will have transferrable free supercharging, the other will not, because they were ordered on different days.