What both you and your friend have done is made an adjective (hardcore, calm) into a noun through the process of nominalization. In this case, both of you use the adjective form to specify a group of people who have that quality: the hardcore (people), the calm (people). We see this in many established idioms and proverbs:
Blessed are the meek, for they shall inherit the Earth (Matthew 5:5)
How are the mighty fallen! (2 Samuel 1:19)
Both the meek and the mighty describe a group of people who have that quality. These usages are often not recorded in dictionaries; calm is does not appear as a noun referring to people in Merriam-Webster. However, if it describes a person, using an adjective with a definite article as a noun is one way to generalize about a group of people:
I believe only the calm survive.
The bold may win the day yet.
There are the quick and the dead.
One other thing to notice - all of these examples use plural concord, that is, the verb agrees with a plural noun. Saying "I believe only the calm survive" denotes an implied plural subject corresponding to the verb. (Example: "the calm people survive".) If you had said "I believe only the calm survives," then the verb would suggest a singular subject, and might refer to various singular meanings for calm.