It is not right to say impossible because it seemed impossible for them to beat Liverpool 3-0 at Anfield, but Atalanta managed to do it. In difficult matches, some teams are more concentrated. Atalanta has something similar. It's like they like difficult matches more. The final match will also be very difficult and if Atalanta is the team that makes fewer mistakes, maybe they will prove everyone wrong. They are doing well in Italy this year. There may be more to come, of course.
Atalanta are a good team from their tactics to team working and they shown it from Serie A to Europa League. They don't have an unbeaten matches like Leverkusen but they don't have too worse quality if we compare quality of two teams.
Leverkusen shown they are strong and stable during this season but they also shown their weakness in many matches. In Europa League, we can list their matches with Qarabag and Roma and I don't know if they let Atalanta score goals, lead 2 - 0 in the final match, will Leverkusen be able to come back again in the most important match in this season with them?
Quality between two teams is quite balancing and this gives Atalanta good chance enough to beat Leverkusen. It's interesting if Leverkusen can finish this season with 3 titles and without any loss but it is interesting too if Atalanta can beat Leverkusen.