This is one of those perennial aviation legends[0] that are still alive, like Amelia Earhart's wreck. Always another rich guy funding a search with promising results.
I remember him saying that the RAF buried Spitfire engines, so it's definitely a real practice.
You may be shocked to learn that practices such as this exist in the modern US military as well.
Although 9 days later:
> Archaeologists believe no Spitfires buried in Burma