This wasn't caused by not reviewing the code of a dependency. This was a core maintainer of xz, who gradually gained trust and control of the project, and was then able to merge changes with little oversight. The failure was in the maintenance of xz, which would of course be much more difficult to catch in dependent projects. Which is why it's so impressive that it was spotted by an OpenSSH user. Not even OpenSSH maintainers noticed this, which points to a failure in their processes as well, to a lesser degree.

I do agree that it's unreasonable to review the code of the entire dependency tree, but reviewing own code thoroughly and direct dependencies casually should be the bare minimum we should expect maintainers to do.