He’s pitching better in September than any other month this season per advanced metrics like FIP BUT he pitched his worst two months in July and August so it’s both.
edit: actually, he had a lower FIP in May so September is his second lowest FIP month of the season. He wasn’t very good in April so to my point I don’t see a strong correlation between his workload this season and effectiveness. He’s been all over the map month to month but generally speaking he seemed to be most effective when his workload wasn’t limited.