The camera negative was 1.33:1. Perhaps it was intended for matting, I don't know.
What I do know is that Pete and the people at the BFI have been doing this professionally for many, many years. Personally, I trust their decision to present the film "full frame", and know that it wasn't arrived at without good reason.
I have a bad feeling about this but I get the feeling asking for more info would lead to some testiness.