Those people are stupid.
There's a few types of gamblers fallacy & they are misperceptions. When you beat a boss you get a roll of the dice, if there are others in your group they also get to roll their own dice so to speak. What they're telling you is that there is one roll of a die for everyone and that the more people there are the higher the numbers go on the dice making your chance smaller.
Your last sentence is correct, and people that tell you wacky theories are the reason Vegas casinos make money, unless they're run by Trump, while people go home poor.