UCL is a prestigious competition, all participants are the best clubs in their respective leagues, as well as Atalanta who were crowned Europa League champions after defeating Leverkusen. Atalanta is a new participant at this different level, they are ready to give maximum resistance, the ambition to qualify for the next phase is the main goal, a tough opponent is waiting. Facing Arsenal in the first match is clearly a tough challenge, but here I will not say Atalanta can win, but if targeting a draw it can be said to have a chance.
Usually a chance to participate in the champions league for those who aren't regular appearance in the champions League, it's regarded as an honor because how els would they have an opportunity to showcase their skills on a level ground with bigger teams of a very high world class standards outside their domestic leagues if not for the champions League and that could be one of the reasons to its prestigious nature. I don't know what it feels like you have regarded Arsenal as a stronger opponent than Liverpool against Atlanta in the champions League, I want you to remember that Liverpool is even more consistent in the champions league than Arsenal and are even more experienced than Arsenal so they are the biggest for me, if it were to be in premier league then I would consider that really differently.