I don't think that's true though. Kate's wedding day tiara had been worn multiple times by Princess Anne in the past.
The Lotus flower tiara that Kate has also worn to a state dinner was worn by Viscountess Linley on her wedding day etc.
The lifetime loan is a loose rule.
Princess Anne wore the scroll tiara when she was very young, before she had use of the Festoon tiara and her grandmother’s Meander tiara
Viscountess Linley wore the Lotus Flower as it was, at that time, Princess Margaret’s tiara. In the same way Zara and Autumn wore tiaras associate with Princess Anne
Generally the British royals don’t interchange the tiaras around in the way other royals do