It's probable, though, that the signal is now signed and encrypted and thus not easy to hijack. Then again, satellites were also broadcasting syndicated shows and news feeds in the clear back then, and probably now, for things that aren't premium channels. It was assumed that if a signal was coming down from the right satellite at the right time, it was the right content.
That said, even back then, it was non-trivial to hijack that signal. There's only a few points of entry: In the station itself, inserting a microwave signal that's closer to the tower or maybe more powerful than the one on top of the station, or in the satellite feed (which has a couple of points of entry, as well, but that would be interruptible by the station master control operator). The microwave transmitter would have to have line of sight to the tower, further complicating matters.
The article suggests that the station employees were able to "stop" the second hijacking quickly...I'm not sure how that would happen if it was a microwave hijacking. They maybe could have boosted the microwave transmitter output at the station to push the imposter down into noise, but those are regulated, I would think, and probably don't have much wiggle room.
I dunno. Even as someone that has worked in TV I don't know exactly how they did it, which makes it all the more fascinating that they did. The simplest thing I can think of is getting into the station and faking all the noise to make it look like it's "merely" a microwave hijacking. Then again, microwave is not terribly advanced technology, it is available on the surplus market, and if you can get closer to the tower with your transmitter, you don't need a whole lot of power. So, that's probably how it was done.