No casino is perfect. There will always be at least one person who'll say "don't play there" because they had an issue, or was unhappy with the service. But that's OK, it doesn't make that casino, if truly legit, a "scam".
I believe the "not providing proof issue" was because it was the provider that forwarded the issue to the casino, so they can't provide any information, OR they can't provide any data/proof because giving it away would help nefarious entities in navigating around the casino's security measures.
Some things are better to be left to the annals of history, or because it would simply do more harm than good, or because parties didn't agree to share such an info.. there many probabilities here.