Sure you should have tests, sure you shouldn't copy paste code you don't understand and you shouldn't push directly to production.

But, regardless of all that, the main issue of all this incident is not the rookie mistake itself, is how they didn't have logs or alerts and it took them 5 days of customer complaining to find out they had "duplication errors" in the db.

That's the thing that should have been fixed first and extensivly mentioned in the post-mortem