I really don't think we do contradict the text. In the beginning of Chapter 18, Fingolfin comes to that conclusion (hey, let's attack Morgoth!) specifically because he sees that the hosts of Men who have moved into his realm have seriously bolstered the numbers of his army. That...hasn't happened yet. That's at least six generations of Men in the future. And that gives the Fëanoreans a reason to turn down the idea - they don't have armies of Men backing them like Dorthonion and Hithlum do. And people become a lot more complacent about peacetime after 400 years of it, so the Noldor thinking that Morgoth can't beat them makes more sense the later into the story we get....though obviously Morgoth has more might than that. Just as Turgon will eventually turn aside from the true purpose of Gondolin and cherish it in its own right, but right now he's all about Ulmo's message and the mission he received from the Vala.
So, aside from being a question for Season 5, I think that Fingolfin could have a general trust in the Siege while not being overconfident that Morgoth will *never* break through without contradicting this text. We are going to show his despair in the aftermath of the Dagor Bragollach - he had to have some hopes that were dashed for that despair to make any sense. Sure, those hopes could be tempered by fears, but some part of him thought they'd managed to keep Beleriand safe from Morgoth prior to that battle.
So, aside from being a question for Season 5, I think that Fingolfin could have a general trust in the Siege while not being overconfident that Morgoth will *never* break through without contradicting this text. We are going to show his despair in the aftermath of the Dagor Bragollach - he had to have some hopes that were dashed for that despair to make any sense. Sure, those hopes could be tempered by fears, but some part of him thought they'd managed to keep Beleriand safe from Morgoth prior to that battle.