Tom Sykes (great-grandson of a Baronet and from a well-connected and royal connected family) stated in print that he'd heard the story from three separate people who he considered reliable sources and that he'd first heard the rumour during a dinner with a woman he knew who was the daughter of an Earl
Well, sort of.
He wrote in the Daily Beast that a gossip magazine in the US had reported on the story and wrote a speculative article peppered with "The palace denies" and such statements.
Over a year later, he wrote an article setting out his "qualifications" to write on royal and social matters for the Daily Beast setting out his family's royal connections which seemed to have waned over the generations. Tom Sykes' main connection seemed, from the article, to be that he missed being at Eton with William by a year.
In that article, he said he'd heard the rumour from "the daughter of an earl" at a dinner party rather than he'd read it in a gossip magazine. It's certainly a better story and more likely to impress his readers.
I've read - and enjoyed - "What Did I Do Last Night?" in which he plays down his family's "grandness".
He also amusingly plays up his willingness to do anything for a story.
I'm inclined to take his picking up the story solely from the D. of an E. with a pinch of salt.
I suspect it was initially because of the US magazine and he may or not have gossiped about it with his friends.
Despite that, I think it's unfair to say that a recovered addict can't be a reliable source.
I do think, however, that a gossip columnist will present things in the most titillating way.