Interesting article, but regarding this specific point. Are you then suggesting to cast the list into a set before checking the membership of an element to improve performance? (Not sure if that would improve much the performance tho, because the casting comes with a price…). If this is the case you should be a bit more clear in the article.

Otherwise it doesn’t make much sense to say that is an anti-pattern since as you well mention list and set are not interchangeable, set is useful only for when expecting unique values, and that doesn’t happen very often.