just take a closer look at the analysis https://www.openwall.com/lists/oss-security/2024/03/29/4

then try to understand the pattern. they backdoored by modifying the build process of packages. now consider the $XZ is also from a backdoored build and the call recognizes in the same way with parameters --robot --version and the shell environment with the hint "xz_wrap.sh" from the piped process. a lot stuff to recognize for the $XZ process that it run as part of a kernel build.

Maybe they put advanced stuff in a backdoored $XZ binary to modify the kernel in a similar way they modified lzma based packages in the build process.

because in order to put backdoor into xz executable, you need to infect its sources. and in order to infect the sources, you need to use a similar technique to hide the modification