It's not. That's my point. Even if Giuliani had the slightest inkling that she was 15, why would he engage? It's exactly the same thing as when a minor claims to be 18, or a rapist claims that the victim said they were. And like my point about To Catch a Predator; the bait actors were adults that were petite and young looking. The men that showed up had every intention to have sex with a child, even if the actor was an adult and they didn't know it.
Forget about to catch a predator for a moment. It's not the same situation. You are having trouble thinking straight because of words.
Imagine being in this situation. A 24 year old woman comes up to you and starts trying to seduce you. She tells you that she is 15 and you immediately know she must be crazy. You cannot help but to feel sexually attracted to her, despite her obvious lie. You are an old man and may never see action again, but you feel so awkward about the presentation, and morally conflicted on how to proceed. "Let's take it slow," you think to yourself. "It doesn't make sense, but as a good lawyer, I will get to the bottom of this."
It's not the same as "Oh hey, you are 15? That's great! I love fucking 15 year olds."
"She said she was 18!" Is very much not the situation here, as you should be able to see.