Channel 4 probably considered they could use the results as fact and that they would be protected by hiding behind the likes of good faith or the reasonable belief that they were true.
What Channel Four reported as fact was that a survey of more than 1000 Ryanair pilots revealed that nearly 90% of respondents said that 'Ryanair didn’t have an open and transparent safety culture'.
Without evidence to the contrary, a court will take the view that 90% of the pilots surveyed did give that response. Whether those pilots were telling the truth or not is irrelevant. If Channel Four believe the survey took place and that the views expressed by the pilots were those reported, then legally they are on firm ground.
Ryanair would only have a case if they could prove that Channel Four knew that the survey was untruthful yet still reported it in some attempt to defame or discredit them.
As I said earlier, there is no way this programme would have gone out without it being completely cleared by the channel's lawyers.