The case in the article was that kosher oil was used today. The concern is that non kosher oil was used in the past. That oil would now be נטל"פ. This is comparable to the תורת חטאת vis a vis the nuts in Krakow.
However, it is highly unlikely that non kosher oil was ever used. לא מיבעיא in countries where all vegetable oil is considered kosher, but even in USA where there is no such policy, the primary brands seem to all be kosher. There is no incentive for a popcorn stand to use a niche oil.