Interestingly, looking at Gentoo’s package, they have both the github and tukaani.org URLs listed:
https://github.com/gentoo/gentoo/blob/master/app-arch/xz-utils/xz-utils-5.6.1.ebuild#L28
From what I understand, those wouldn’t be the same tarball, and might have thrown an error.
Came to post the same. Seems like the most awkward possible way to phrase that.
Your “Disks not included” suggestion, or heck, just “empty” would surely be better.