Have a 1 year project. One side is a team of 10 developers with 5 years of experience in the field. Other side is 1 developer with 5 years of experience in the field. Do you think it is unreasonable that there is any developer that could beat the team in that situation? The quality expectations for both are the same, so no shortcuts.

I don't find that unreasonable at all, most teams are very unproductive. So what I'd call it is "A 10x developer is a developer who can outperform a typical team of 10 developers".