You have it exactly backward.
Fox advertising dollars (their ONLY income) is based on ratings (number of viewers). As you will see later this morning, it was a huge ratings FAIL!
You will soon learn that this has cost them dearly ($millions)
I think what Fox (Ailes) said was that the ads were already booked and priced before the event (I’m guessing Fox convinced the advertisers to hang in there because they were ‘working’ with Trump to get him to appear - that’s the reason for the three Ailes phone calls to Trump).
So, Fox managed to keep the scheduled spots by wangling until show time and got their money. Where it WILL definitely hurt them is in any future scheduled debate, if any, because the advertisers will demand cheaper rates.
It really all boils down not to how many viewers did last night’s debate get in comparison to the others, because that’s an indicator of what Fox can get away with as to ad pricing.