I don't think the author meant that you needed to be 100% proficient at the job on day one. As in his example the goal is to avoid answers like 'CEO of Google'

I don't think the author meant that you needed to be 100% proficient at the job on day one. As in his example the goal is to avoid answers like 'CEO of Google' At least, if I was asking the question, I would be encouraged that you want to expand your skill set.