Is it a sin for a strict novus ordo presider ordained in the new rite to offer the Traditional Mass?

This is a good question. The only way it would be a sin is if he was not really a priest because the new rite of ordination was invalid. I am undecided on that question. If he was not a real priest then I think it would be a sin. But the priest would only be culpable if he knew that he was not a real priest and said Mass anyway.