This is gonna sound like it's a bit and that I'm just parodying Peterson's way of speaking in vague hand-wavey style but I promise you that it's not and I'm legitimately making an argument here...
To Peterson, the historicity of The Gulag Archipelago is irrelevant to him because it represents a higher order, metaphorical truth.
When you grasp this about what is important to Peterson's approach to truth then it becomes very obvious that he plays fast and loose with facts because he's not concerned with mundane truths that are the domain of historians and scientists, in fact he holds them in contempt especially when they run counter to his understanding, because he sees his purpose in uncovering and elucidating the metaphorical truths that structure our existence.
If this sounds like a bunch of nonsense woo then you're right. If this sounds like the attitude of an aspiring prophet then you're right.
But if you want to really understand how Peterson's brain works and why his avid followers seem largely immune to facts then this is the mentality that you need to wrap your head around. Pretty much everything that Peterson does is a sort of exegesis on this higher order, metaphorical truth that he's both attempting to understand himself while attempting to articulate it to his audience at the same time.
There's a quote from him that really illuminates his belief about his "mission". I'll try to find it and if I do I'll edit it into this comment.