Interesting that the article refers to the "American secret service," the "US security service," and the "surveillance arm of the US," but fails to mention any US agency by name. Also they wrote M15 for MI5, which is just lazy. Anyhoo, Diana was no longer royal, wasn't performing any official business for the UK government, and wasn't in UK territory at the time she died. Does that make her fair game for espionage? Perhaps. But I've got no idea what information she could possibly have had. Maybe we were actually spying on the Fayeds? That would be just as strange.
As for Diana.....maybe they were afraid of the same pillow talk that got Marilyn in trouble.... ;)