The beginning of the story of those $50:
Karas drove to Vegas with his car and $50 in his wallet. After arriving at The Mirage, Karas recognized a fellow poker player from Los Angeles and convinced him to lend him $10,000.
So, the bottom line, is that he turned $10 000 in 4 million by simply winning against a single player, he didn't gamble as playing in slots he played against another player with deep pockets who lost all his money to him. Same for all his other winnings, he played against other players, so his luck was not just about his hands but also about meeting and agreeing to play with enough high rollers with deep enough pockets for high rolls but also not in their best form.
Not skills but luck. Gambling is all about luck only. Skill is something that can be improved by practicing, but in the case of gambling, what matters is just good luck.
He played poker, poker is not just luck, you need to know how to play otherwise there would be poker tournaments!