The idea is predicated on autonomous control and presumably the vehicles communicate in real time...they all brake together at the delta V of the least capable member.
I don't actually know how long it takes between the computer deciding to brake and the car actually starting to slow down, but I expect you can't just throw out a message saying "I'm braking immediately" and start braking without any kind of confirmation that the message was received, and expect it to be OK.
But maybe you can afford to wait for a confirmation? The speed of light is pretty fast, and you're not transmitting that many bytes.
Especially if you consider that a human could easily take 200 milliseconds to react to something even if it's really paying attention. You could request a braking confirmation from the other side of the Atlantic and still come out ahead!