This reeks of an inside job - someone not only had access to Aramco's email system but also knew that LG Chem was supposed to be making the payment, so the list of suspects is probably pretty short.
We've not actually seen the email that LG Chem claimed they received, so we just have to trust that the letter did indeed supply a new account number. OR maybe, just maybe, it might have started like this:
"Dear LG Chem,
You may not believe your luck, but I am His Royal Highness, Sheikh Abdul al Abdul and I have $773 millions dollars in a Nigerian bank account and I urgently need your help..."