'we did it in a safe way'
They had no control over the cars behind, anything could have happened.
Their attack could have been demonstrated on a parking lot or private road.
Their information did need urgently sharing, and while I understand the common need for a bit for drama, this was utterly unsafe & for the good of everyone I hope they've pulled their heads out of their asses since making that video!
I don't know any vehicle with connectivity (other than Jeep and maybe Ford?), which does not have safety critical CAN/FlexRay buses segregated from driver facing 'infotainment' systems.
What that means is that the network bus in which your 'compromised' infotainment system is able to operate is completely separate from Engine, ABS, AEB, ESP, Airbags etc.
The solutions vary but there is usually a physical gateway that prevents a passthrough MITM attacks, so you cannot simple send a message frame from your infotainment pretending to be an AEB module requesting emergency braking to your ABS system.