I think that it is a mistake to call this a UI issue. Its not like a mistake was made because text was difficult to read, or because to check boxes were too close together and the wrong one was checked. Those would be UI issues.
This was an issue of an operations team that just did not know how their system worked. Perhaps the system design could have been better. Most likely it could have been, but if Citi had a team of people using a system responsible for billions of dollars without the competence to use it correctly, that failing goes way beyond the UI.
I have to half agree with you, but in essence disagree. You are absolutely right in that this is not a UI issue where a checkbox was too small. But it was an usability issue that was caused by an inadequate UI. Plus an inadequate process, but that also manifested in the UI. UI issues can cover a lot :)
The user thought that setting the target account in one line would be enough to have the money be sent there instead of where it ended up being sent to. There the UI was misleading, that's a violation of the self describality a dialogue has to fulfill (I'll freely translate the principles, I would have to look up the official english translation); in that the UI has to communicate by itself in which state it is, what every button does, every icon means, what even is clickable etc.
Then the system did not clearly communicate what would happen. That's not only an issue of describing itself, it's also an issue of fault tolerance. The system did not prevent the user from a mistake he was about to make. This could have happened by clearly describing what would happen at the end of the process and offering an undo operation. They tried to implement this outside of the UI by requiring 2 additional set of eyes, clearly that was not a sufficient solution.
But most importantly, the process the UI was offering was the wrong one for the job. This was about sending parts of a debt to creditors. Instead of offering a clear way to do that, they had a convoluted process that involved a wash account plus a real money transfer to that account (how crazy is that!) instead of supporting exactly the work that was to be done. That violates the primary dialogue principle: Be the right tool for the job (Aufgabenangemessenheit). Admittedly, I miss context to know for sure what would be the right tool for the job and why they ended with this solution, maybe it's a bit less crazy than it sounds.
Yes, all of that together goes far above simple UI issues, but it's nonetheless an issue of the UI. It's just also a complete usability fail. It's really the job of usability professionals to analyze software and scenarios like this and to provide better solutions. That was and partly still is my job :)
The headline ideally should have called it an usability issue. That would have covered the UI part and included the other aspects.
This was an issue of an operations team that just did not know how their system worked. Perhaps the system design could have been better. Most likely it could have been, but if Citi had a team of people using a system responsible for billions of dollars without the competence to use it correctly, that failing goes way beyond the UI.