This is how it is being used presently:
https://github.com/twitter/hraven/blob/master/hraven-etl/src/main/java/com/twitter/hraven/mapreduce/JobFileTableMapper.java#L176

The intention of this check in is to retain the same behavior.

The parse method should be called once per job history file. There should be a job history listener per job history file. I don't think multiple threads would be parsing the same job history ever.