A good understanding relationship does not go kaput over a deleted SNS post.
If they can't sort thru something as silly as that then it is better they call it off now than later. They have always said they understand each other.
I sincerely doubt there is any big issue between them. Maybe she is just stepping back so that the limelight stays on him and not their relationship.
But a lot of it still doesn't make sense. She shouldn't have left the comment section open on her cousins post in the first place. I hope she does not close off the comment section for the YouTube post. If she does not like certain comments she can block that person or delete those comments. No need to close the entire comment section.