I don't think it's plausible OSS-Fuzz could have found this. The backdoor required a build configuration that was not used in OSS-Fuzz.
I'm guessing "Jia Tan" knew this and made changes to XZ's use of OSS-Fuzz for the purposes of cementing their position as the new maintainer of XZ, rather than out of worry OSS-Fuzz would find the backdoor as people have speculated.
Had this not been discovered, the backdoored version of xz could have eventually ended up in the ubuntu version oss-fuzz uses for its docker image - and linked into all those packages being tested as well.
Except now there's an explanation if fuzzing starts to fail - honggfuzz uses -fsanitize which is incompatible with xz's use of ifunc, so any package that depends on it should rebuild xz from source with --disable-ifunc instead of using the binary package.
And if that's the case, it was sloppy of "Jia" to disable it in OSS-Fuzz and not do this:
``` __attribute__((__used__,__no_sanitize_address__)) ```
to the XZ source code to fix the false positive and turn off the compilation warning, no attention would have been drawn to this at all since no one would have to change their build script.
With or without this PR, it's very unlikely OSS-Fuzz would have found the bug. OSS-Fuzz also happens to be on Ubuntu 20. I'm not very familiar with Ubuntu release cycles, but I think it would have been a very long time before backdoored packages made their way into Ubuntu 20.
There's some plausible evidence here that they may've tried to use alter egos to encourage Debian to update the package: https://twitter.com/f0wlsec/status/1773824841331740708