I know cervical spinal cord injuries are a serious injury, but if a 80 old man could walk with a cervical spinal cord injury straight after the trauma, would that mean his cervical spinal cord injury was not life-threatening before he walked?

Any general movement after a neck fracture can take an cause injury or amplification of an existing injury to the spinal cord. The standard of care after an event where neck injury is possible involves immobilizing the neck in a neutral position before that person is moved.

Yes, without doubt. Good Luck

Immediate neck trauma does not necessarily cause morbidity unless an upper neck lesion threatening the phrenic nerve function, so-called C-zero issue. But an unstable spine contacting the spinal cord can create additional cord damage secondary to walking with additive post-traumatic inflammation and swelling, eventuating in increased neurological damage.