To build castles in the air means to daydream, but in this sentence she is being realistic about the two of them and what they could mean to each other.
Context: He couldn't let her be realistic about the two of them and what they could mean to each other.
Hypothesis: He couldn't let her build castles in the air about the two of them and what they could mean to each other.

Is the hypothesis incorrect? (Yes, no, or unknown)?