I don't think they were detained. That has a specific meaning of not being free to leave, where it seems here as if the police merely asked questions. The police don't need permission to ask you anything, although you're not obliged to answer either. However, we'll never know exactly how this went down legally speaking, since the top poster avers that he had his headphones on for the first several minutes of the interview.
In the specific example of a traffic stop, I believe you are considered to have been detained. You are correct that merely asking someone questions isn't the same.
Though to be honest the whole story doesn't make sense.