It's for building embedded Linux distros, and your typical Linux distro contains quite a lot of C++ and Rust code these days (especially if you include, say, a browser, or Qt). But you have parallelism across packages, so even if one core is busy doing a serial linking step, the rest of your cores are busy compiling other packages (or maybe even linking other packages).

That said, there are sequential steps in Yocto builds too, notably installing packages into the rootfs (it uses dpkg, opkg or rpm, all of which are sequential) and any code you have in the rootfs postprocessing step. These steps usually aren't a significant part of a clean build, but can be a quite substantial part of incremental builds.