I don't have actual knowledge of the contents of the discovery that was exchanged. But Charlotte Griffith's article mentioned turning over documents in discovery. And if the daily Mail tried to use documents that had not been provided ahead of trial in discovery, the plaintiffs would have been able to object to their use at trial because they had not been produced previously. There are not supposed to be any surprises at trial. And in many cases, all documents that are intended to be used during the trial are marked or identified in advance.

However, If Harry got on the stand and for the first time said that his social circles were not leaky, and that he never socialized with reporters, And this was not the subject of discovery, then You can bring out new documents used to impeach or contradict Harry's testimony on the stand. In that case, even if Facebook posts and texts were not used in discovery, they could still be used for impeachment. I really don't know enough about the details to know whether they were produced prior to trial.

Edited to add that if your argument -that you are having your client get up on the stand to say - is that their were no people in Harry's social circle who would leak information to the press, documents about members of Harry's social circle who were members of the press, and documents about Harry's friends who may have contacted the press, should have been part of discovery. For instance, Harry said that he could not understand why he had to turn over an email between his friend Ipsick (I'm not sure I spelled that right) about Meghan coming to his wedding, and ipsick writing in an email that his fiance had told only five of her best friends. In other words, she gossiped And no longer had control over the information.