The failure to roleplay stats is a social contract violation. When you agree to play the game, you agree to abide by the rules unless they are changed. The player gets to decide how his PC thinks and acts, but only within the parameters the game sets up. The game sets it up so that social skills cannot be forced on the player, so the player gets to decide. The game also sets up that stats mean certain things.
Page 185 is not being overridden. It simply doesn't apply to social contract violations. No rule can enable such a violation.
The game sets it up for the player to be advised to consider the meaning of these things, but to ultimately decide for themselves how it informs their character's appearance and personality. For a particular portrayal to violate the social contract, as you say here, it must exist as an agreement at the level of table rules. People should be held to their agreements, but the game rules don't actually say what the player
must do here other than take what it says into account when deciding. A group that lacks this table rule has no issue with a social contract violation when a player decides to portray the character as Sherlock Holmes while having an Int 5, for example.
Now, if we want to talk about personal characteristics (personality traits, ideals, bonds, flaws), this is where the game actually
does say something about how the player might receive a benefit for portraying the character in certain
pre-established ways via Inspiration. A character with an Int 5 might, if a player decides, have a trait or flaw that says "I'm as dumb as a Heward's handy haversack full of light hammers." Portraying the character as such during play may then earn that player Inspiration.
As has been mentioned already, it's not particularly smart play to try to have an Int 5 character attempt to make deductions or recall lore as Sherlock Holmes might, since it may result in a lot of failure. But that's the player's choice to make.