Just peruse the Android git tree. We're not talking about syscall tables for POSIX interfaces. Those sorts of things constitute a mere fraction of the total of their tree. Most of the tree defines interfaces to Linux-original subsystems.
Yes, most of the unguarded sections are meant to be used in user-space. That's inconsequential. That's like saying all of your header files for your libfoo are not copyrightable. What does it matter what it's going to be used for? What makes something copyrightable is inherent in the content, not the authors' intentions for it.
And the question of how "thin" your copyright can be in headers doesn't amount to much, either. Can there be copyright in "struct foo"? "struct foo; struct bar;"? "struct foo; struct bar; struce t baz"? Google is saying it doesn't matter. No code in headers is copyrightable, period. Which amounts to saying that copyright can't subsist in the definition of your API. Which begs the question, if an API isn't copyrightable how can you prevent derivative use of that API? You can't. Not until your code is linked and loaded and running with the implementation would it be derivative. Which means there's no functional difference between the GPL and LGPL, if what they're doing is legal.