I don't know if this lesson has tips¬es or not, but if not, then it will probably be added in the future with an explanation to this.
The prefix adds a perfective aspect to the verb, which now implies that the action will be completed: we will end up on the second floor. If you wanted to express that we are hurrying down right now without implying any result, you could leave out the prefix: "Sietünk a második emeletre." Though "down" is missing from this sentence -- we could be on the ground floor and hurry to the second floor from there. To make it clear you can add "le" after the verb: "Sietünk le a második emeletre." or "Sietünk lefelé a második emeletre." (lit. "downwards")
This explanation makes sense. The next lesson has the following sentence: felrepülunk mi is. This is translated by: we are flying up too, and not by: we fly up too. I am not sure about the implication now, and if here the present continuous (are hurrying) should be rejected (it is currently) or accepted as an alternative answer.