I think everyone has different definition of casual and hardcore. I personally do not associate casual related to playtime. I define it along the lines of how dedicated a person is to the game.
For example using gears. A casual, my definition, would just look at certain stats. It has higher primary stats or it has higher item level. Therefore it is considered an upgrade. A hardcore would understand that the secondary stats also matters. Does it help to reach a soft cap etc.
So a hardcore to me has done additional research, homework if you like, into what is best for their toons or play style.
No unlike sports. Some just play football. While others would spend additional time training to get better at it.