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The argument is that it's just _interface_, which has to be that way for interoperability, so has no "creative content". Code that is actually expressive code in a header file (inline functions, or non-constant macros) could readily spoil this. I have no idea whether the output headers contains more than structure definitions, constant macros, and forward function declarations.

Almost. It's non-copyrightable because it is required for interoperability, and is therefore fair-use under US law.

Pretty much. They also threw out a few other elements, but I looked at some of the original header files and the Android equivalents and it was hard to find any difference other than the removal of comments.