I assume it from experience that comes with many times with many different players deciding that my hypothetical efforts to hash out problematic elements into acceptable ones and the response about that feedback being claimed that the backstory was "denied" with a vow to "But I will return the favor: your world's history and lore will mean nothing to my PC. If you expect me to care about your world, I expect you to care about my character." spotlights the fact that the assumption was entirely justified
Let me point out where I find the breaking point for me is.
As a DM, I set up an initial premise. (Eberron pirates, Curse of Strahd AP, etc.)
People come to me with an idea. (Thorin).
I advise what elements might need to be changed to make it work (Thorin became a pirate or got sucked into Ravenloft, etc) and what that will mean. (Thorin might have to wait until he leaves Barovia or gathers enough gold pirating to raise an army).
If these edits are fine, we move on, if not, the character might have to be shelved until a more suitable game comes up.
Lather, rinse, repeat for each player.
What I don't do:
Assume the player is doing this to subvert my campaign.
Outright reject the concept unless it is such an anethema to the central concept of the game it is unreconcilable (a very rare event)
Make my players roll to see if they can play said character (sorry Thorin, your Dex score is too high. Maybe you want to be Samwise instead?)
Allow other players to veto ideas of other players (I won't adventure with a dark elf. Change your PC)
As to the quote: you are looking at my retort to the notion that who my character is doesn't matter. My character's goal, identity and origin will never matter in your game. If the answer is "no, you cannot be Thorin because your origin doesn't matter and I won't do anything with your plot hook. Create a new character." I will reciprocate with "I don't care about the background of your world either, give me some orcs to fight and pie to win." Apathy cuts both ways.