Nixos has it easier here because they don't require packages to be "installed" before building code against them. For Gentoo, none of their build scripts (ebuilds) are written to support that.

"treat it as a cross compilation scenario" is essentially what the post discusses when they mention "use a different CHOST". A CHOST is a unique name identifying a system configuration, like "x86_64-unknown-linux-gnu" (etc). Gentoo treats building for different CHOSTs as cross compiling.