Yes they weighted the seats, slipped out from a seatbelt, and weighted the steering wheel.

Newer software released in the last couple months also adds some kind of gaze tracking using the interior camera - because the car now alerts constantly if I am looking at my phone and will disengage auto-pilot and put me in the penalty box if I keep doing it. I don’t know how that tech would react if it didn’t see a human in the drivers seat at all, but I imagine ultimately that will be another safety measure. Tesla has a lot of incentive to cut down on these types of pranks.

For those that don’t know, disengage keeps autopilot running but with an extremely annoying alarm and massive red warning on the display, and as soon as you take over the penalty box means you can’t re-engage “AutoPilot(TM)” until the next drive.