It was just amusing to see such a formal engagement labeled as the opposite. It can be a good idea, beneficial for both parties, etc. But if a candidate can fail it, it's certainly not normal or informal.
For clarity, candidates fail the lunch slot not by chewing with their mouth open, keeping their pinky on their cup, or putting elbows on the table, but rather by being overtly bigoted, racist/sexist, or otherwise extraordinarily A-hole-ish. (I can only recall two instances in 14 years; perhaps I should have not mentioned it, but I'm an engineer at heart... :) )