I'm guessing that it invokes /usr/include which on Linux is probably not neutral to CPP or gnu-isms.
Your point about "compatible" is well made. I should have been clearer.
Wasn't gentoo born as Linux userspace on BSD kernel at one point?
I think Gentoo was always about Linux with BSD-style ports. They had some additional "flavors" that switched Linux to kernels from FreeBSD, NetBSD or OpenBSD, but it seems those alternative Gentoo distros are deprecated for at least a few years already.
Gentoo was born as linux. Portage, the package manager, was inspired from BSD ports. Maybe they tried to experiment with also using a BSD kernel later, as did Debian, but it was only a side project.