The Liverpool incident last season has turned out an eye opener to many others that winning the Europa is something any team other than the bigger teams can do, interestingly this is one of the rare occasions as Atlanta isn't a regular winner of the Europa, they even made a record amongst their Italian mates winning the Europa last season. Sevilla didn't qualify for the Europa so I think there's a mix-up with your reference. Although Sevilla happens to have the most Europa trophy and if they were to be in the league this season with their experience I believe the would have been able to do something interesting.
Not only Liverpool, even Leverkusen who were highly favored at that time had to experience defeat, and what was even more surprising was that it was their first defeat and the only defeat they felt last season. I'm sure at that time everyone was on Leverkusen's side, except for those who were Atalanta supporters, even if Atalanta supporters were realistic by looking at the statistics of both teams they would also predict that Leverkusen would win.
But this is football where everything cannot be predicted precisely and Atalanta proved that they could beat an undefeated club and they did it in the most important match, namely the final match. This is one of the factors why football is a sport with the largest fan base in the world. Here it is full of surprises, no matter how strong the club is, if they have to lose in the match, then they will lose.