To Peterson, the historicity of The Gulag Archipelago is irrelevant to him because it represents a higher order, metaphorical truth

And to add on to your comment, I think Solzhenitsyn himself sort of takes this approach in writing. I’ve heard Gulag Archipeligo described as “campfire stories about the gulags”, and I think that’s very apt. It’s not factual information, it’s just him gathering stories he hears from others. Whether or not those stories are true isn’t relevant. Solzhenitsyn saw it as his mission to bring down the USSR (and replace it with some Christo-fascist authoritarian state) and whatever stories he needed to pull out of his ass to make that happen was fine by him.