It doesn't matter if you are unranked, bronze, or plat. In normals the average elo of each team is about equal. If there's a diamond vs a silver in one lane, then another lane has a bronze vs a plat. This doesn't make it fair, but the elo of each TEAM is balanced.
So if you're truly playing vs a challenger - first off I'm pretty sure they wouldn't waste time on a normals match - then you or someone on your team has challenger elo rating whether you've ever played ranked or not. Or if not that, then say 2 of you have master tier rating and the next highest ranked person on the other team would have elo equivalent of diamond. There's plenty of people though who use accent marks, etc. to make it look like they have the exact same name as a challenger or pro player - which is a far more believable possibility than that you are actually facing a challenger in a random normals game.