There were actually 3 broadcasts, and if you find the third, on youtube, you will quickly realize exactly how it was done. Take note of the type of the network on all 3.

I thought that was due to terrestrial overpowering.

Yeah it was, but the problem is the same.

The assumption that inability to get the transmitter or knowledge of how to do it is enough to protect your reciever.

As far as I know, relay satellites also did not use authentication in the past.

That was in the days of analog TV. No good way to authenticate uplinks there, I think.

Subscriber satillite would have existed by the time of the incident.

I don't think it was a technical limitation and it's likely encrypted uplinks would have been possible long before consumers got encrypted downlinks (subscriber satillite).