You refer to this as a tarball, but it is not; it is a compressed disk image designed for booting in place (rather than running an installer, hence “preinstalled”).
Inasmuch as http://iso.qa.ubuntu.com/qatracker/milestones/433/builds/248085/testcases and http://iso.qa.ubuntu.com/qatracker/milestones/433/builds/248084/testcases point to the riscv64 and raspberry pi versions of the preinstalled images respectively, it appears that http://iso.qa.ubuntu.com/qatracker/milestones/433/builds/248086/testcases ought to be the place to track the corresponding amd64 preinstalled image. However, “Ubuntu Server amd64” quite clearly refers (in its download URL and its test cases) to the discontinued d-i based server installer image. I do not know why “Ubuntu Server” was used as the name on the ISO tracker for the preinstalled raspi/riscv64 images, leading to this confusion, rather than giving them a distinguished name.
Cc:ing @sil2100, who added the amd64 and arm64 preinstalled images to the build. I do not know why we are building these images if they are not testable and not releasable, as is currently the case; we should either get them on the ISO tracker, or stop building them.