I think it's all to do with money.
There were rumblings in Canada that Invictus sponsors were unhappy that the event was turning into the "Harry and Meghan show" rather than celebrating the veterans. Especially since the disabled competitors have to individually fundraise their own travel and accommodation while Harry and Meghan fly in on private jets, are put up in 5* accomodation with security and Meghan changes outfits several times a day. Two of the top chiefs of IG Canada were allegedly sacked for voicing concerns that that the Sussex's expenses were putting a huge strain on the IG budget.
Therefore I expect that potential sponsors are demanding William and Kate, and the King in attendance as a condition of handing over the cash - like the glory days of 2014 where the Charles and William attended for several days, but let Harry take centre stage.
So leaking these "invitations" to the press is just an attempt to yet again manipulate the RF by attempting to blackmail embarrass them into attending and making to clear that Harry won't be fobbed off with Prince Edward.