"Isn't Sky News on Freeview? Really don't know but thought it was?"
Even if it is, not everyone has had the digital switchover, so there may well be pensioners who have BBC 1, 2, ITV, C4 and Five (if they're able, even that isn't available across the whole UK on UHF). Those pensioners have no immediate need to get Freeview, and would be unable to see the Sky News debate.
Therefore it seems a perfectly reasonable question.
I suspect the answer is that with Sky having several million customers, they can easily claim to be news source number 3 on TV, and a growing source for (off-peak) news on (commercial) radio, too.
Has been a while since I bothered with TV news but I assume C4 or Five sometimes do their own news, but may have completely switched to using news from ITN or Sky.
Certainly many commercial radio stations must be paying for news output from the Sky News Centre as it means that (overnight, for example) they just switch inputs for 2 minutes and play what comes off a satellite feed from Sky, easy as that.