You probably could get airborne without reheat, if you could garentee no engine failure, but it would be a massively gutsy move and the take off roll would be massive.
If you watched them they will have run the engines up to max dry then lit the burners one at a time and allowed them to stabilise once lit before releasing the brakes. They take a finite amount of time to light up and start producing thrust and they don't always light.
Both of these would pose an unacceptably high risk if the take off was carried out without both burners already lit at the beginning of the take off roll.