For me I don't see any connection between gambling and demonic manipulation, well one thing I realized about most people is that whenever they dislike something they always look for every angle to convince people that is not good to venture into gambling, in as much as gambling has an influence on people but I seriously doubt if there is any demon in it that always cloud the judgement of people into aggressive gambling.
Though I wouldn't blame him because most people because of religious beliefs sees gambling as a sin, but for me I don't believe on such thing because if gambling should be a sin that means indirectly sports would be regarded as a sin because is from sports gambling is derived, so anybody that told you gambling is a sin he is obviously getting it all wrong but however I would have actually believe the guy if he had say that gambling has affected many people negatively.
Personally, I don't think so, as well, because the gamblers are responsible for their actions. And nothing as such would be affiliated to gambling being made of some superstitious elements. The casino is ran and managed by humans. People who say such things about gambling may have failed in the niche and try to disarm other gamblers with shady words as such.
What people think is between them and it's not relevant to allow ourselves as players to let their words get to us negatively. Provided a gambler has a nice goal, he's meant to focus on it and avoid the opinions of people like the one Op met. Also as players it's our duty to evaluate what people say about the game and conclude if it's acceptable or not.