Translation:We will arrive in the city in two hours.
I saw the link. It does not say that my answer is wrong, rather it says that Duo's answer is right: Which, as I said, I have no problem with.
So no, I continue to contend that my answer is right, especially since the point of language is to convey meaning: If someone says to me that they have arrived in or at a city I'm hardly going to stop to tell them that they're wrong about their preposition use, I'm going to be more concerned with wherever it is they've arrived at after all
Both are synonymous in written, spoken, colloquial and idiomatic English and among the many synonymic translations Duo accepts, my answer should be one of them.