My explanation :

Hardcore is an approach to the game that the majority of other players may feel is 'not needed,' 'over the top,' or excessive. E.g. ,a hardcore player will level their chochobo to 10 in 3 weekends. He will set this as his goal, and focus on that goal until it is complete. A casual on the other hand, may take 3 months to achieve the same goal, or not consider this as a goal at all.

Player skill is a subset to the hard-core mindset and not necessarily a defining quality. I say this because hardcore players tend to grind more, and repetitions typically hone a skill.

Lastly, time is irrelevant. A hard-core player may have had lots of time in the past to play games like this. And while that excessive time played in the past probably developed his approach to gaming today, he may not be afforded the same luxuries to play all hours of the day like within his previous life .