I've used Gentoo for almost a decade now, and no, that's not true. emerge doesn't just randomly keep source files on disk, and certainly not in a form easy to link to. In fact, Gentoo is worse than Debian for static linking, because not all packages have IUSE=static-libs. If it doesn't, you need to patch that ebuild and potentially many dependencies to support it. On the other hand, on Debian, the standard is for -dev packages to come with both headers and static libraries.