Refuse to answer. It's none of their business, and nobody can compel you to answer a question.
Someone in the article, or someone who took an interim lower-wage job out of desperation may rightly not feel quite so empowered.
One needs to consider it as offensive as, "Are you planning to have a child soon" or "How old are you" or, "You look real purty"
Also if a company is going to invite you all the way to their office and use up their current employees' time, it would be a very stupid thing to not want an applicant just because they don't want to discuss their current salary.