I just said that it's a civil offense but skipping your court date is a criminal offense at the federal level.
So, if you're ordered to appear in federal court, even for a civil penalty, and you don't show up it can be treated as a crime. Which is what happens in immigration cases. The NTA is a federal summons, it's served in accordance with the rules the immigrant agrees to when they get their visa. When they disregard the NTA, it can be treated as a separate criminal offense. So now there are 2 issues in play: 1) the civil offense for unlawful presence; and 2) the criminal offense for disregarding a federal summons.