There is a risk
That is to say, you would (almost certainly) be breaking Spanish law and the law where you live (whichever actually applies) to the extent that the company could sue you and win if they chose to do so.
Reading between the lines, you ordered a service which they delivered and you paid for and which you say was delivered “poor”. I will make an educated guess that there were some ToS provided by them which means with near certainty:
- The contract is almost certainly subject to Spanish law plus any non-excludable consumer protection laws of your country (assuming that is where the service was delivered)
- You aren’t entitled to a refund, at least, not a full refund. A full refund is only a legal requirement when they have totally failed to provide their consideration (the service) under the contract.
It is clear that you and the company had a dispute over the contract. Whatever the legal merit of your position, the company made you an offer to settle this dispute. This offer was conditional on you keeping it confidential. In accepting their offer you accepted their condition and formed a new contract that includes a condition of confidentiality. If you break that condition you can be sued for breach of contract.
Notwithstanding, even if you had rejected their offer, the fact that they made it was confidential and communicated to you in a way that informed you of its confidential nature. In a common law country, breaching this would be a tort: I don’t know enough about Spanish law to know if they have equivalent provisions.
Take the money and keep your mouth shut.
The data protection provisions have nothing to do with the offer, they relate to retention of personal information - they can keep the email indefinitely as it relates to a business transaction.