It's possible though. The noise around it did at least put Freund on alert and we should be very glad both that "Jia Tan" made the mistakes they made originally and that Freund followed up on their gut feeling
One wonders whether the xz backdoor would have been discovered if slightly less obfuscation was used.
The whole xz incident is a pretty strong argument to:
a) change practice from including binary (opaque) test files themselves to human-readable scripts and tooling that build test files on-demand,
b) raise suspicion of any binaries included in open source projects, and
c) create much more scrutiny around dependencies of 'highly scrutinised' packages like OpenSSH.
It's a shame that there isn't a foundation (that I'm aware of) that can donate time and effort of vetted developers to foundational open source projects like xz.
But xz is not a dependency of upstream OpenSSH you see. It was a dependency of a patch created by Linux distros for systemd integration.
Video of Jia Tan fixing the valgrind bugs: https://www.youtube.com/watch?v=A16YuzuKN58&t=138s
If a state actor (it almost has to be a state actor at the time frame they were operating under) could put in this much effort once, they clearly could afford to do it X times. And when you look through the history of communications from the author, it just reads like 'another day at the office'.
Outside of Valgrind bugzilla bug reports these claims almost never stand up to close scrutiny. Not that the people making the claims ever perform any scrutiny. It's usually "my application doesn't crash so it must be a false positive" or "I'm sure that I initialised that variable" or "it's not really a leak, the OS will reclaim the memory".
