I agree with you. UCL and UEL are on different levels. Atalanta were impressive in the Europa League and even managed to beat Liverpool who often play in the Champions League. But now Atalanta are really playing in the UCL. I am not sure Atalanta will be able to do the same as they did last season. All the top European clubs are in this competition. They will always be motivated to win the Champions League.
The two competitions that are being discussed here are all European continental club competitions but there's a reason why one is regarded as the "Champions" League while the other is called the Europa League. The European Cup which is now known as the UEFA Champions League competition is not just the biggest club competition in Europe but the in the whole wide world and it's contested by the strongest European clubs that ever existed so as expected, it's miles away stronger than the Europa League.
Atalanta did exceptionally well in the Europa League last summer where they went on to win the competition after beating the competition's favorite Bayer Leverkusen in the final which automatically gave them a place in the UEFA Champions League this season. Atalanta if you ask me still have all it takes to have an incredible season in the UEFA Champions League competition this season but they'll need a miracle to win the competition since there are other stronger teams in the competition.