That’s a rather paltry article to link to declare “fiction”, based solely on his ex-wife. From wikipedia -

Solzhenitsyn constructed his highly detailed narrative from various sources including reports, interviews, statements, diaries, legal documents, and his own experience as a Gulag prisoner.

en.wikipedia.org/wiki/The_Gulag_Archipelago

It’s not “100% accurate, detailed notes from one man’s point of view”, but this book is widely acknowledged to be a nonfiction account, even if many of the tales & details are anecdotal and not verifiable.