Another trap of a question like this is complacency. The second that you start labelling yourself as good, you lose your competitive edge.

I disagree. It's an opportunity to exercise self confidence. This type of question is too subjective to provide a meaningful metric. Before you sell yourself short, how do you know their definition of "expert" is the same as yours?

So, if you think you can actually do the work, rate yourself a 5. Screening candidates is their job, not yours. Your job is to get hired. And so what if you jump in a little over your head. It's good motivation to improve your skills.