Exploiting what? There wasn’t any software hierarchy for Teletext to escape from in analogy TVs. If you found a bug in the Teletext chip you couldn’t then go on to do anything to the TV set aside print different data to the screen. And since you’re already tuned into that radio frequency and controlling the data sent on it, you already have control of what’s sent to the TV screen already anyway so who cares?

As an aside, I once interviewed one of the guys who wrote Teletext processors for analog TVs. He was a very interesting individual.