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Swift is secure in itself. Although the basic message is indeed a text file and you could format one if you want to, try entering their secure VPN to send that message. Only banks have configured terminals that are supposed to be secured and locked away from unauthorized personnel.
There is definitely some human intervention somewhere. Although I believe that the malware can allow you to make some changes from an external system that interfaces into Swift, messages sent there pass thru manual verification and authorization process (so called four-eyes). There were insiders, no doubt. Took advantages of weak controls.