Which shows that the word is undefined. But I would expect, whatever the actual details, "hardcore" means unsuitable for broadcast TV. I would agree that GoT might be defined as porn, but is being broadcastable automatically makes it not hardcore. My definitions would not include anything transparently consensual as hardcore, but explicit portrayal of sex is porn. But the "hard" in "hardcore" implies some level of violence or coercion.
Anyway, I introduced the word into the conversation, and what I means was the sort of non-consensual violent porn which I think would be damaging to children. Whatever the words used, there are some extreme images which are capable of damaging children. While I accept that consenting adults should be able to access such stuff via moderately protected channels on the internet or similar, they should not, as the OP suggested, be transmitted free to air on any wavelengths the transmitter chooses, including those already in use for domestic TV. There is a need for a regulator of some sort - though the rulebook for that regulator is not obvious.