It's also a rocket launch, which is commonly accompanied by a countdown.

It was also pretty common to have that countdown broadcast over the radio, as many people would show up just to watch the rocket launch. Even without a radio broadcast, it'd be as simple as a walkie talkie inside the control room and another with the filming team.

This shows up on the internet every few months, and I'm still astounded by how much attention it gets. It is not nearly as remarkable as portrayed.

Learning it wasn't composited surprised me. I always assumed it was green-screened (or blue-screened or whatever they used for chroma-keying back then) - that's because until ignition, the camera focus makes it seem like he's standing in front of a sheet with a painting on it, and overall, it feels as a very well-made composite.

Looks like a completely different take. If you look at his microphone it is worn higher in the first "walking" shot than in the takeoff shot.

Also the walk is filmed South of the Vehicle Assembly Building looking North. Whereas the Voyager 2 took off from LC-41 which is East of the VAB

and the exact time of launch is known beforehand, so it's not like he was guessing and happening to get lucky with it lining up.