"Provided" headers mentioned by Florian are generated by a script - specifically, scripts/install_headers.pl. Which is to say, "remove sections under ifdef __KERNEL__, disable #warning in linux/types.h, strip annotations and some qualifiers, remove including of compiler.h".
AFAICS, they've done that unifdef - __KERNEL__-only sections are removed. If anything, they are stripping it harder than install_headers.pl does.
They might want to throw "known to be useless for userland" headers, excluded by make headers_install. Because any userland code that changes behaviour from exclusion of those headers is severely broken and they want to catch that anyway.
IOW, if all that stench amounts to "they are not using the headers provided by kernel for libcs", they can trivially switch to feeding results of make headers_install to their script, thus excluding the same files and reducing namespace pollution. At which point it's down to "they updated their libc, be very afraid!" Pathetic.