I agree with everything that you said, Madrid has all the superstars in the world from the history and is the strongest in European competition. However, Atalanta only has average players that looks very strong due to their current form. When analyzing on a football match, you check if the teams are in form or not. Real Madrid is not in form like previous seasons which gives Atalanta the chance to win the match, because they are in form.
Apart from what you said, I actually hope their match can be fierce even though Real Madrid's performance has not been good so far. Because Ancelotti will definitely find a way for his team's performance to improve a little when facing Atalanta in the Champions League because Madrid must be able to win in three matches in this phase. And Atalanta themselves will also try to perform as well as possible in front of their own supporters so this could provide a level of fierceness in their match later.
Real Madrid's inconsistent performance as we have seen in the last few matches, I have serious doubts whether they will be able to do well against Atalanta. There is no reason to consider Atalanta a weak team this season. This team is in a very strong position this season. The top spot of the Serie A table is occupied by Atalanta. Atalanta is undefeated in the Champions League so far. And the team is in fifth place in the Champions League.
On the other hand, Real Madrid's overall performance is inconsistent. Madrid has not yet been able to occupy the top spot of the La Liga table. Madrid's performance in the Champions League is very weak. Madrid's position in the Champions League is in 24th place. In such a situation, winning the away match against Atalanta will be a big challenge for Madrid. I think Atalanta will win or draw in this match. Madrid's chances of winning are low.