Debian has also had Debian GNU/Hurd in unstable state for years. That Gentoo/FreeBSD page mentions that the port is "experimental" and "incomplete". So presumably they're talking about a non-Linux kernel as a first-class component in a stable release.
Admittedly, Gentoo's port uses the FreeBSD libc whereas Debian ported glibc, and the Gentoo port is not exactly ready for prime time, but claiming Debian did it first is a bit of a stretch.