"You guys are American."
You guys is not 你们. That's just 'you'. If you want to go colloquial you'd say 大家 or 朋友
It's not about colloquial or formal, the creators are using "you-you guys" to contrast between singular and plural, like "你-你们“. Agreed, it isn't the clearest of solutions and can confuse some, but it is the best they can do with English which has lost that distinction (although I personally think "y'all" might be a better, more transparent option - the course does accept it in answers).