Page 15 of the PHB:
"In the first tier (levels 1-4), characters are effectively apprentice adventurers. They are learning the features that define them as members of particular classes, including the major choices that flavor their class features as they advance (such as a wizard’s Arcane Tradition or a fighter’s Martial Archetype)."
I see so the Sorcerer's magic, metamagic, sorcery points, and first bloodline ability are learned?
And if the sorcerer's abilities aren't learned, is it not reasonable that other abilities might not be learned? After all a warlock and cleric are granted powers as well, are they not? Rage is a learned ability?
But, you will likely call those exceptions while the example of cunning action isn't an exception, because of reasons you will likely try and make abundantly obscure.
Fluff, though, doesn't exist in a vacuum. It has to not only match the mechanics you are dealing with, but you have to be careful what your new fluff says, because it can also imply OTHER mechanics which if they don't exist, can cause disconnects.
Take Chaosmancer's hesitation re-write of Cunning Action. Because he uses hesitation as the reason he doesn't have cunning action and the loss of that hesitation as the reason it just appears out of nowhere, he creates a mechanical problem with my PC. My PC has the same Int and Dex as his PC, so either I am forced by Chaosmancer to have a perpetual case of hesitation, which is no bueno, or my PC who doesn't hesitate should automatically get Cunning Action, despite not being a rogue.
The same goes for the cyborg with extraordinary fine motor control and extraordinary vision. Such a PC would also have bonuses to perception where vision is concerned, as well as seeing oncoming enemies before they can see you, giving the PCs an advantage when out in the open.
Those two pieces of fluff are poorly written and cause disconnects when applied logically to the PC and the rest of the world, since the other implied mechanics will not be present.
But your character doesn't have the same background as mine, the training, the reason for his hesitance and the core of his identity in the fiction we have created, which sets up the fiction for him to actually be more highly skilled than he would present himself as.
My fluff seems "poorly written" only in the fact that I did not feel like posting the entire backstory and concept to be eviscerated as breaking something or other that you have deemed important.
After all, it isn't even a bit of fluff that the rest of the party is likely aware of. I didn't tell them this after all, because no one asked me "why do you suddenly have cunning action as a level 2 Rogue?" but
I wanted an explanation that made sense for my character, so I created one.
And, if the implied mechanics are that anyone can learn any class ability from level 1 to 4, then why can't an 10th level wizard decide to learn cunning action? Seems your fluff also has implied mechanics which will not be present