My question is what would have happened if the tank didnt have a burst disk. My guess would be.....nothing.
It takes a doubling of ABSOLUTE temp to double the pressure in a tank.
So if the tank had 300bar in it from an over fill at 20degC the tank would have to reach 180deg C or so to get to 400bar. I very much doubt a good condition tank would rupture at 400bar. I also doubt it would pass to many more hydros.
My understanding of burst disks is that there were more designed for liquified gas cylinders and for protection from tanks in fires. Europe do not use them for scuba.