I wouldn't dismiss a candidate for not knowing how a clearfix works. What's important is they understand positioning and layout and how things tend to fit together. I'd reasonably be able to extract this information from them. Asking about clearfixes is one of many ways I'd go about picking their brain.

What I would dismiss them for, however, is the cocky attitude that they never need to bother learning any "old, bad practices." I'd dismiss them for thinking they are "above" working for a company that supports technology that was released just 4 years ago. Further, I'd dismiss them for the attitude that their solution solves all the problems without considering potential drawbacks. These are all huge red flags that show they may technically be knowledgable, but are very difficult to work with. Not someone I want on my team.