The fchown() system call supports passing -1, cast to the appropriate type, as a no-op value. The systemd people were attempting to wrap similar semantics around fchmod(). Originally in 2014 M. Sievers specified (mode_t)0 as the no-op value, which wasn't a good choice, with M. Poettering changing it to (mode_t)-1 in 2015 but overlooking one place where the value remained tested against 0.

* https://github.com/systemd/systemd/commit/c38dfac9ed6c1c3beb...