The pull request says the check confirms it was started as privileged Bash by reading the caller's command line. The installer runs through arch-chroot, which starts that command in a new process-number space while the process list is still the outer one. The shell's own number then names no process, and each privileged entry point exits with status 126. Thunderbolt authorization is the first of those entry points during install.

The merged check runs in a child of the caller and reads that parent from its own process entry, in the numbering of the process list it is looking at. Outside that case, the parent is the same process as before. emirb opened and merged the change. emirb wrote in the pull request that on a nightly edge virtual machine the Thunderbolt setup script exits 126 as shipped and exits 0 with this change. A full install was not run.