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As I recall, the Double Diamond Triangle Saga is only claimed to be non-canon by the fans. But it's more accurate to say it's out-of-continuity: it's apparently set in 1377 DR, way ahead of the setting when it was published, and features characters who were later killed in books set in earlier years. But a simpler explanation is that the only date given in all nine books, "the Year of the Haunting" is in error and that it's more likely set in 1367/1368 DR.
Whenever I get around to covering the DDTS as part of my Utter East project, it will get more attention.
I think I'm probably in the minority in that I actually liked the DDTS and wish that it was canon. There isn't enough coverage of the Utter East and it's a shame that WotC dropped its canonicity.
That being said, it wasn't just the fans that declared it non-canon, WotC itself apparently decided to just write the whole series off as an “Apocryphal Future Fiction of Volo’s published in Realms”, granting the DDTS the unique status of being apocryphal canon in the Forgotten Realms. The place where they explicitly did this no longer exists on the web, but using the Internet Archive: Wayback Machine https://archive.org/web/ and plugging in the old WotC link to travel back to an article originally published around 2005, you get: https://web.archive.org/web/20050212151736/http://www.wizards.com/forgottenrealms/FR_Timeline.asp#index
From there, click on the link that says, "Apocrypha / Volo's Attempts at Fiction", and the DDTS books are listed.