For starters, that 90% is based on nothing so lets not jump to conclusions about who plays what.
Secondly, since there is no actual definition of what hardcore and casuals the actual number skewing doesn't matter in the slightest. It could be 99 to 1 and still be accurate to how I perceive it.
Also, what you said was "Doing +15s is not casual", implying there is some form of skill limit to what casual means. Since you genuinely need to have an idea of what you are doing in those key levels and higher, otherwise you you get punished by the mechanics.
If you spend a few hours a week since the beginning of the expansion doing just m+ and gated content to boost your character you could very easily be doing 15's. It all comes down to how good you are. Some are inherently worse at learning than others, but being a good at a game does not make you hardcore.