A “hardcore” game might be described as one with only a single source of uncertainty: player performance. No random factors. Complete information. Only how well the player performs is in question. Whether that is dexterity, timing, coordination, strategy, tactics, or all of the above, performance uncertainty is often sought after as the “ultimate gamer’s uncertainty”.
Performance is driven by the player making the right, or best, decisions whenever a decision comes up. In theory, tic tac toe is still a game, because the players might make incorrect choices. In contrast, it’s very hard to determine exactly what “performance” means in a tabletop RPG. While it’s clear that games like Mario or Dark Souls are solely dependent on the player’s performance, a good tabletop RPG doesn’t have a clear line of “winning” or “losing”.
So we have to look at the actual goal of an RPG: to have fun. That might seem like the goal of most games, and it’d be nice if that were true, but the truth is that tabletop RPGs are specialized in having fun. There is no other goal, and no other way to “win”. Therefor, a player’s ability to perform well at an RPG is purely based on their ability to have fun at the table. We could even go as far as to say that outstanding performances even help other players have fun.
Ignoring all other systems and rules, our ability to have fun playing an RPG is certainly an uncertainty. But having fun at the table is a skill that you can be good at, you can work at, and you can become better over time!