Yeah it's frustrating as hell. I have the 502VM with the XG Station 2, but after finding out that it's the JHL6420 controller, I gave up on buying a video card for it, for now.
I even contacted the Asus support center about replacing the controller and I got informed that that is not an option... and I think that's the least they could do, cause I think they are getting a lot of bad rep from this situation.
The splash page for the laptop has fine print about different SKUs having different controllers, but I've yet to see a 502/702VM with a proper TB3 controller.