There's a logical "skip" between that and
>Eventually it learns how its unconscious operates, and how to interrogate it, and its post-hoc justifications, at least in the common cases, become reliable.
The brain constructs a narrative that won't be called out as false, one that provides social capital, makes one feel good about oneself, is consistent with all your other justifications, etc. It's only an assumption that this process would naturally converge on Truth, and considering it's massively-multiplayer chaos where brains coordinate their stories in complex ways, my assumption is that this would converge on *stability*, not truth.
(Even if you are incentivized to lie and flatter yourself, it is still helpful to have access to the true signal internally, because that way you can know how to structure your lie to best avoid detection.)
I mean, no we don't, both in a personal way and in a global scientific understanding.
What you're saying happens is a set of socially consistent and acceptable responses based upon general human knowledge at the time. The common cases aren't exactly reliable, it's that they are repeatable in the sense they cover what we expect, and tend to explode when the world is less predictable.
This is why the scientific method changed the world, because we started writing shit down, comparing notes, and striving for repeatability.
Frankly, my opinion is that DNA is incredible at choose the most energy efficient/cheap option, and the cheaper option is definitely justifications afterward.
I feel strengthened by psychological experiments where people are shown fake events involving them, where they then "explain their (nonexistent) reasoning at the time".
Arguments for the idea that the human consciousness/soul is something that is emergent keep getting shouted down though. Even though if you take the extreme opposite: it's obviously wrong. Nobody has ever cut open a human skull (or anything else) and found a soul. So somehow it's constructed from very non-conscious components we don't understand, it's not "actually there" in a real sense.