It actually depends on how you define an arrest. One imagines that an arrest is when you are being told that you are placed under arrest by a police officer. However, aren't you technically under arrest when a police officer pulls you over since you are not free to go on your way until he tells you to?
Yes, technically you are being detained.
But the point is that the N-400 interview is not a law school exam, they just want to check if you have good moral character, I don't think they're trying to trick you.
Anyways, that's what he said and he sounded pretty confident and knowledgeable.