Yes, as has been said multiple times, swapping in part of a different shot from the same day to get a group where everyone looks good has always been fine.
Adding a person in who was not there has never been OK.
It is being suggested with both photos that additional people (Kate, possibly the Queen, possibly several of the children in the shot with the Queen) were never there. That has never been acceptable.
It’s the equivalent in the link above of William having flatly refused to go to the wedding (obviously didn’t happen) and the photographer using a photo of him at a wedding 6 months earlier to pretend he was there. If that had happened, the photograph would be an outright lie.
It is apparently very easy to prove that photos are tweaked (what is normally meant by photoshopping) rather than fabricated to show an event that never happened (what various conspiracy theorists are claiming has occurred). For some reason KP are refusing to provide the evidence which is allowing the conspiracies (both about what happened and why) to flourish