This isn't a Burst bug, because the same thing happens with Burst disabled. And also, I don't think there's a bug here. The error message says correctly that you must call JobHandle.Complete() before disposing the output data used in the job. And I think the user was aware of this, because the sample project contains a commented-out line that does the correct thing: generatorHandles[0].Complete();.
Note that you must call Complete before accessing the NativeHashSet Capacity, because the job system has detected that the NativeHashSet is used in the job, so it's only safe to access it after the job has completed.