This is certainly true, which is why professional historians cite their sources, preferably original sources (not the opinions of other historians).
> Fiction writers can write deep truths
Yes, they can. How can you tell if they're writing the truth, though?
You're right, I don't like Sinclair's obvious and heavy-handed socialist agenda. But do you ever see me citing fictional works that promote freedom?
> it threatens your version of the truth
My version of the truth is based on reading verifiable historical examples of what happens under socialism, and what happens under freedom. Not novels of either.