I can think of several reasons he wouldn't have said anything at the time, can't you?
For the sake of her poor children for one, probably a sense of duty towards the royal family / decorum, for it being seen as spiteful and strange to discuss it so soon after her death, to take the focus away from his anger towards the media which was his agenda at the time. He made some quite pointed anti RF comments at her funeral.
I'm not saying it's true or that he's even a credible source, but not saying anything public at the time is not a reason to disbelieve him.