That makes sense, but there remains preponderance of evidence from multiple sources.The focus Peterson places on documentary evidence is to cut across gaps in memory thst cone over the decades. A given person's account if what happened 30 years ago in a dispute is by the nature of things fuzziness than contemporary court statements. We all conflate and edit over time.
Consider for example, a comparison of memory vs a written diary from the time. The diary could be given more weight because it was closer to the time in which the evidence was gathered, but there's no guarantee that what was put into the diary at the time was inaccurate.
Then there is the 'sources' problems we see again and again in modern media. Multiple outlets quote the same source and, some will weigh the outlets as cumulative evidence when, there is just a single evidentiary source.
Here's something that comes to mind when it comes to sources and RPGs. I was at a 'con and looking at a vendor's monster book based on Japanese mythology and folktales, and the writer happened to be there. I asked him what his sources were. Ultimately, he admitted that he just used other RPG books.
I think a good takeaway is that there's nothing wrong with making a statement but, if you want it to be taken seriously beyond that of a personal experience, you need to back that up with sources, and the sources themselves need to be adjudged their evidentiary value. There's nothing wrong with an 'Old Salt' relating their experiences - just accept that its a statement of a memory of a personal experience.