I don't know what photo of Archie is referred to above.
The story of the hair extensions has been twisted somewhat, it seems the Palace were asked if it was a scar and the Palace confirmed it was.
"We sent the photos out to some of our celebrity hairstylist friends and their immediate reactions were: "Those aren't extensions."
no extensions on the right side! O'Connor and her team speculated that it looked like Middleton could have a scar
Turns out, O'Connor was right! The Mail confirmed with Clarence House, which handles press for both the Duchess and Prince William, that the mark on her head is a scar, resulting from an operation in childhood:
“A spokesman for the Duchess at St James's Palace confirmed last night that it was the result of surgery as a young child. The scar related to a childhood operation,' they said. The palace declined to discuss what kind of operation because it was, they said, a private matter."
https://www.glamour.com/story/kate-middletons-extensions-how