To expand on this, Pirsig tears apart "just" as a completely useless word which reaffirms the speaker's belief but also trivializes the listener's inability to understand. "just" comes from ancient Latin "ius" and it means "I swear" or "I believe". When somebody says "I was just doing something", they are swearing a light oath that they were doing something.

Back in context, what the parent is saying is that the grandparent provided a "just-so" story: The parent finds it easy to believe that history happened in a way which neatly lines up with the story that they are telling. Did earlier humans know about perspective? Grandparent believes that it's obvious.