Well, not totally fictional. During the compilation process the header file of readline is combined with the source code of the program and the result is processed by the compiler into a single object file and eventually into an executable. Unless you want to suggest that the contents of the header have no influence at all I don't think you can suggest the binary is not derived from the header file.
The reason your windows programs is not affected by this is because the licence of the relevant files is such that you are specifically allowed to use them to compile your programs and have no effects on the licence of the result.
That case you refer to doesn't really apply since it's talking copying (reverse engineering) an interface for compatibility, whereas we're talking the literal textual copying of file with an explicit licence. You are not being forced to use the readline interface.
[Be careful to separate two issues: PostgreSQL is not a derived work of readline, but any PostgreSQL binary you compile using the readline headers is.]