They are dangerous, correct, but this is definitely not the Atletico Madrid from five years ago. They have always been infamous for their amazing defense, but that's quite some time ago. It doesn't mean there would be any good reason to not take them as seriously as in the past, but still I would be shocked if they were to beat Inter Milan. On that level it is of course possible that they have a good run in their game at home and score first, but if that doesn't happen, I think Inter Milan is one level too high and too clever and strategically too strong to still give this away.
I know it could turn into one of these ugly games that Diego Simeone loves so much, but Inter Milan can be quite ugly as well if needed. They know how to play this game. But yes I can only recommend everyone to get ear plugs for this one as well as Simeone will scream the hell out of his body!
He is probably the only one who can compete with Jurgen Klopp when he is freaking out. I would love to see both of these coaches one day make their most aggressive faces and stand head to head, I think everyone knows what that means.
Atletico who think their look strong at home actually have to make mistakes in the first leg by playing defense, honestly I think Atletico Madrid can even have a chance to be able to equalize the score or even beat Intermilan in that game if they do not defend completely, like you said, Atletico now is not Atletico a few seasons ago who did have a balance between their forwards and also the backline, For now I also don't think if Atletico will be able to overcome the attack carried out by Intermilan later, let alone they are burdened with 1 goal on aggregate.
If we compare Atletico then and now it is very different, Atletico who used to often park the bus when indeed they were superior both home and away, while Atletico now precisely every time they play with a team that is at the same level as them at the opponent's home, they will choose to defend and just counterattack to keep the ball away from their defense and we can see this happening in some important matches Atletico.
In the second leg match maybe Atletico will try to attack although it is also possible that they will still defend fully, but when I see Intermilan who are still playing impressively in the league when they are close enough to the Scudetto and without caring they continue to play attacking when facing their opponents, and that's why 1 goal aggregate may not make Intermilan defend and will instead continue to attack Atletico to add to the aggregate victory in second leg match.