You, sir, are technically correct. Except in the case where it's a noun, according to the American Heritage Dictionary
hardcore
n.
1. The most dedicated, unfailingly loyal faction of a group or organization: the hardcore of the separatist movement.
2. An intractable core or nucleus of a society, especially one that is stubbornly resistant to improvement or change.
3. often hardcore (härd'kôr', -kōr') A form of exceptionally harsh punk rock.
And we both know it's colloquially used to mean hardcore gamer in the circle this website is devoted to.