I know and agree it is a definition that may sound strange initially, but it does have a firm meaning/intention.
For my part (and others who have been flight simming as a hobby for decades) “hard core” simply means dedication to flight simming by acquiring knowledge and experience through use of proper flight rules and laws depending on the aircraft and area where one is flying in/out of. Furthermore, it is the acquisition of skills/experience/knowledge of how to fly a particular aircraft or several aircraft according to real world procedures and following them accordingly. This is all then restricted by the actual sim itself in terms of what the sim environment can bring in real terms like weather, and a multitude of other real world factors and how they influence the actual aircraft. Not withstanding, devs producing aircraft that mimic their real world counterparts as closely as possible also allowing for any sim inadequacies. Hard core simmers are demanding on all devs and always asking for the envelope to be stretched further and faster for greater realism.
Then there are the casual type persons who only enter flight simming for a short time and/or who are only interested in browsing around the sim very casually. And there’s no problem with that to be clear.
But therein lies the difference between “casual” and us “hard core” simmers.