It's definitely what I would consider the basic expectations for a build system (why would you even need root to compile an OS?), but it's not something that most non-embedded-focused distributions deliver.
Usually it's to make it easier dealing with file system ("disk") image files, which may require root privileges to mount and use with regular programs (though FUSE and similar subsystems can help.) In a mounted filesystem, root privileges may be required to set permission bits like setuid.
The NetBSD toolchain probably includes utilities to make it easy to copy data in and out of file system images, ideally concurrently and without corruption.
In addition to building filesystem images, a common use of root for operating system building is to chroot into a "clean" environment to avoid environmental corruption. (In general, chroot is reserved to uid 0 due to issues with setuid binaries -- on some systems chroot can be made available to non-root users by turning off setuid behaviour.)
Yeah, I feel like anyone making this statement hasn't done any sort of wide cross-compilation in the past, let alone of an entire OS.
It should be a basic expectation, but most certainly isn't one in the real-world. Especially with stuff like GCC2 Haiku, which requires an old obscure GCC version for building ABI compatible images.
