So why did his legal team go to all the trouble of finding those articles if they're not relevant?
I think it’s the difference between being a claimant and defendant. Harry’s lawyers were looking to put a case together. He says “there are loads of articles about me that must have come from phone hacking” So he and they go through the hundreds - thousands - of stories published by the Mirror over the years and Harry identifies the ones that, in his words, seem suspicious. His team wouldn’t necessarily spend the time and money doing a 360 degree review of each and every one to see if they can be undermined at the time they pick them. Because there’s a long way to go, and a lot of water to go under the bridge before you get to a trial, if you ever do.
The defendant however gets served with a claim and they want to defend it. They are very motivated to try and find a way to undermine each and every allegation, even if that means spending huge amounts of money chasing down what was being written and said elsewhere around each individual story. And they have an advantage - access to the journalists who wrote them in the first place who might have more insight about what went on.
This would all have come out at an earlier stage, of course, and normally that’s the stage where the claimant would look to settle their claim. Harry, it seems, was not interested in doing that, hence you have to box on and take your chances at trial…