We don't actually know if it was Diana in the car at all
Where on earth does that nugget come from? The photographs clearly show Diana in the car before and after the crash; the medical staff worked on Diana and the two pathologists cut up Diana. The undertaker popped Diana in a box.
Unless there was a switch made on the plane on the way back, it was a very dead Diana in a box that arrived back in the UK.
I can only hope that those sort of posts are an elaborate joke and not another manifestation of the ludicrous conspiracy theories that have dogged the poor woman's passing.