The Linux kernel licence explicitly says programs using the syscall interface are not considered derivative works and that GPL does not apply to them: https://github.com/torvalds/linux/blob/master/LICENSES/excep...

nolibc is NOT under GPL. See first line of each file.

/* SPDX-License-Identifier: LGPL-2.1 OR MIT */

It's technically not part Linux's headers either. It's published under the tools subdirectory, so it's something that ships along with the kernel, but not used by the kernel itself. Basically it's there as some people might find it useful, but could've as well been a separate repo.