Thanks for your comment. When I say "fictional," I mean that persona should be realistic, not real. It means that when you design a persona, you always make assumptions about the attributes that the persona will have. The only way to design without making assumptions is when you use an actual human being as a persona, but it is not what we want most of the time since personas represent segments of users.

The part about making assumptions is a trap many product teams fail into. It's fairly easy to make incorrect assumptions even when you have research data. It doesn't mean that a persona is a bad tool. It is the way we use this tool in product design that really matters.