I'm conflicted, because both of these statements are true.
People are going to come to you with unrealistic requirements. This can't be a discussion- you have to say “No, there's no way my team can pull that off, we need to fix this”, and you have to have the relationship and credibility for them to believe you, trust you, and know they can work with you to come up with a solution. That's ambiguously like being a peer that also has the authority or respect to have the final say.
At the same time, you'll find out that every new project starts with the same response from your best developers: “No, I don't think we can do that”. 99% of the time, there's some ambiguous requirement making them apprehensive at a totally reasonable request. You have to have the relationship to firmly say “I'm willing to put in the hard work to figure this out with you”. You should have the credibility that they will have some kind of faith that you can better scope requirements in a way that alleviates their concerns.