Yeeeeah...I quite like Jim Waterson, but the Guardian's fact-checkers seem to have started the bank holiday early.
The legal argument boils down to this: when did Prince Harry fully understand that he was potentially a victim of phone hacking? And then did he start his legal claim in time?
It doesn't matter when he "fully understood" it. That's not what he has to show in court. He has to show either that his claim was issued within the required timeframe (for some of his claims this is likely to be the case, as some instances are from 2016), or, if not, that he can make out that one of the specific grounds in the Limitation Act for granting an extension exist. "Not fully understanding" is not a ground for extension. He needs to show that News UK deliberately concealed the essential facts of the claim, and he couldn't have reasonably discovered those facts, to the extent that it was not possible for him to make out the facts of his claim within the 6 years.
Sadie Frost won some of her claims with the facts being that an article appeared about a particular private matter, she hadn't spoken to anyone else about that matter, and her assistant had left voicemails about that matter. Harry needs to show that this level of factual information was not available to him because of News UK's deliberate concealment.