The qemu linux userspace implementation for hexagon is recently available and open source, yes. Not sure whether it would've been any harder to do fuzzing with

The qemu linux userspace implementation for hexagon is recently available and open source, yes. Not sure whether it would've been any harder to do fuzzing with the closed-source simulator that's been available for a long time. But the fuzzer doesn't need the libraries' source, it just makes it easier for the fuzzer if it has access to the source. And the availability of emulator or simulator code is probably independent of this too. The 400 distinct bugs are the uniquely faulting instructions or paths uncovered by the fuzzer.