"the existence" of any complaint? Not only can you not disclose the details but you can't even say you complained.
Lets just finish it off for them.... AND if someone asks if you ever complained
you should deny it ever happened.
In fact you should probably deny you even have an account at our casino.
Actually we would prefer it if you deny that you've ever even heard of it.... We do.
Actually no, this would be a lie.
You would actually drop them right in it and not breach the term either. Instead of answering the question, you would refer to this term explaining that you are unable to give an answer to this question as it might break this term. Instead of generating bad publicity because there are complaints, the publicity would be generated about this term, implying that the lack of bad reports about this casino is not anything to do with it being good, but being down to bad reviews being suppressed.
This thread is one example of how the mere existence of this term is generating incredibly bad PR for a casino that should have everything going in it's favour, such as a relatively rare UKGC licence.
Some UK players know that the UKGC licence means it has to operate to UK law, and hence it follows that this term would not be enforceable, so if you broke it, the worst they could do is refuse to resolve the complaint internally. It's also likely to be in breach of their UKGC licence to interpret the term as it's written, as they are REQUIRED to offer an independent third party dispute resolution option.
This term could even be used to request that independent review sites warn users that any glowing reviews of this casino may give a biased view due to this term effectively banning anyone from giving a negative review because something went wrong.
Most players, as we know, would not ever see this term until it's too late, so it's not having it there that will damage them, it's making it common knowledge among the player community that the term is there that will scare away potential customers.