I do agree with you in that by this same logic, you have a better chance of arriving at great self-control if you start with decent self-control.
When I hear "I don't have a say in the matter" I read frustration, not dismissal.
I'd posit that when people say that, they don't mean it literally. Or, rather, it's the best they can articulate the internal struggle. I've found the language I know (English) to not provide me with the tools to describe the nuance of the sensation of lacking self-control or willpower.
What I describe going forward is hard-won from long term introspection, and it still falls short of effectively communicating what I am really trying to say.
I would wager that, for a not insignificant fraction of people, when they tell you they "don't have a say in the matter" they are trying to articulate the feeling/sensation of having the intellectual understanding of the situation i.e. (paraphrased for brevity) "I should do X because it has benefits Y and Z, instead of A which has downsides B and C" but where merely having that understanding is insufficient to compel (willpower, initiative, motivation, w/e) themselves to action, for however long they choose to linger on it in their mind.
> Strength: ... I built my mental strength by running even when I didn’t want to—when I was sore, stressed, or sleepy.
He built his will power by doing things he didn't have the will to do? One of the pillars of his self-control is that he has self-control? Maybe I'm missing something or jumping on the easy mockery, but this seems circular or common-sense.