I would expand on what @DMDave1 said above. The passage you quoted refers specifically to working with the DM to create a new mechanical benefit for a custom background. It does not in any way state that you need the DM's permission to alter class fluff to, for example, have a Barbarian PC who aspires for their tribe to join civilization rather than viewing civilization as a form of weakness.
Having the rest of your tribe agree with you that civilization is a good thing would require DM buy-in by default, since the other tribe members are NPCs, but a PC's personal opinion of civilization is normally entirely up to the player. If a specific opinion of civilization would somehow create problems for a specific game, the DM can totally ask the player to change it, but, absent such a request, the player is doing nothing wrong by unilaterally determining their character's opinions.
of all the fluff texts any part of it that is seen as "determining character opinions" of PCs is to me a case of bad rulebook writing. When dealing with "opinions of the characters" or outlooks or viewpoints IMO they should be stated as "some feel" or "it is common..." and i tend to take it as such when i GM.
like i said - i have no problem with a civilized barbarian and have no issues working out the backstory with the player - even if that working out turns out to be "yeah this is fine".
but lets look at a bit beyond the "opinions of the character."
Consider...
"My sorcerer is actually a studious scholarly type and his sorcerer abilities are not actually derived by drawing on heritage but on a variety of rare incantations and rituals he has learned and performs. So his ancestry fiend armor and so on (all his ancestry features and class abilities ) are not due to a fiendish dalliance but just something he learned to create thru sophisticated magic rituals and specialized mastery."
here, the player generated backstory basically vaporizes the entire fluff for the class and shifts pretty close to that of the wizard.
if allowed, any "distrust" between wizards or organized magics and "natural casters" would not seem to apply - tho other biases could. if allowed, others might seek him out to get him to show them the rituals etc - whether he can or not. if allowed the player has added to the world that the entire sorcerer fluff can be handwaved away and the entire ancestry bit becomes just "one way to get to those effects" and not something linked to them both ways.
is this also a case of "character picks their fluff" one sees as "automatic player choice"?"
is this unquestionably "player right - gm hands off" when it comes to a character for a campaign?
Even the CHA vs INT can be explained with a bit of "compelling bound spirits" or other type of fluff.
Even an INT 8 can be as well... lots of time spent on researching these kept me from other normal studies.
What if the bookish student studying rituals and dark arts came upon the arts that let him bind into himself special magic macfluffies that allowed him to...
insert fighter class stats.
insert rogue class stats.
insert cleric class stats without divinity needed.
etc etc etc.
Does a Gm have any say whatsoever in this being inserted into his campaign if he has not absolutely and explicitly forbade any way it could be fluffed up beforehand in some mega-tome of nots?