If writing is held to the standard of absolute truth (which I think we agree is unknowable), then yeah it's all BS, but so what? We'd still be far worse off without it.
I find truth (of some kind) in poetry, fiction, and every other writing genre. But the reader generally has to dig for it, or at least watch for nuggets of it to appear along the path. And the more a writer insists that truth is what s/he is telling, the more tightly I hold my wallet and my nose.
I just got out of my evening class, "Advanced Forms & Theory of Creative Nonfiction," where we were discussing Kiese Laymon's memoir, "Heavy" and will next read "In the Dream House" by Carmen Maria Machado. So I'm pretty sure all writing should be greeted with a reasonable degree of skepticism, but I also think some bullshit makes for good organic fertilizer.