You’re on the right track here and it does appear that your car suddenly thinks it’s about to crash and triggers the Autonomous Emergency Braking (AEB) system to avoid the phantom prang. And it does that by automatically slamming on the brakes. Again, you’re right when you suggest that if other cars had been around at the time, the car’s attempts to avoid a crash may, indeed, have caused one.
I have a couple of questions for you: Does this problem occur when you’re driving with the active cruise-control engaged? And, does it happen when driving on a downhill section of road that then begins to level out? If the answers bare yes, then you’re not alone, because those are the precise circumstances reported by more than a dozen 2018 Polo owners in the US. The theory is that the levelling terrain is detected by the car’s sensors, causing it to confuse the undulating road with a potential collision threat. Calibration and set-up is critical in these sophisticated modern AEB systems, and something is not right with your car. I doubt that rebooting the system (as the dealer has suggested) will make much difference if the sensors are angled or calibrated incorrectly.
Honda has experienced similar problems with its 2014 and 2015 CR-V model which also had the potential to confuse inanimate roadside objects (like wheelie-bins) with potential crash obstacles, and produced a similar response from the car. Honda has actually recalled those CR-Vs in Australia to deal with this, but Volkswagen Australia does not appear to have followed suit, telling me that it hasn’t seen any cases of this yet (at head office level).
Honestly, I don’t blame you for refusing to take the car back. I wouldn’t want to be driving around in a car that could suddenly, and without any warning or legitimate reason, apply its own brakes as if there was an emergency. I’d be short-cutting the dealer and going straight to VW Australia’s customer service division and spelling it out.