If you read the article at all, it clearly mentions that the traders in question don't believe they bought the shares on margin. Whether or not this is user error, who knows. And Citadel have gone on the record repeatedly stating they did not influence Robinhood's trading halt. The information available makes it seem as though Robinhood were told they needed to deposit more into their DTCC (clearinghouse) accounts to allow trades to continue - essentially, that Robinhood got called on their own margins at the clearinghouse level.
https://www.ft.com/content/9a1b24e6-0433-462a-a860-c2504ea56...
https://www.bloomberg.com/news/articles/2021-01-29/for-robin...
https://www.nytimes.com/2021/01/29/business/dealbook/robinho...