@eater xz is a part of *the build process itself* in many cases - extracting source archives, that sort of thing. So it could have affected the source of other applications at any point in that process, in a way that's impossible to trace back.
So anything that has come into contact with xz at any point in its build or distribution process, while this new maintainer was involved, is now suspect. That's... a double-digit percentage of packages on a typical system, I suspect.