With due respect, that’s bad advice.
The formal complaint is important to escalate the case further. Every financial institution has to have a complaints procedure. There’s usually no point in raising the same case more than once that way, it’s unlikely that one get‘s a different answer.
But If one wants to escalate a case further to financial authorities or the Ombudsman, it’s recommended to provide the outcome of a formal complaints to them. It’s part of the evidence you provide when you want to challenge a company’s „final response“. One can even contact the Ombudsman when a business isn’t responding, but one should be able to proof that one tried to solve the problem directly. And following the complaints procedure is the necessary step to do this.