At first sight that we 'already know everything' seems a ridiculous idea, that has apparently been refuted by the great accumulation of knowledge through human history; but I believe there is a sense in which the broader argument is indicative of a profound insight - and this is why the argument has been taken seriously for more than 2000 years (ie. since Plato); and is intrinsic to the work of Owen Barfield.
The sense is that life is two curves running from childhood to maturity - a rising line of self-consciousness which increases from childhood to a maximum plateau attained at adolescence. And a descending line of innate and spontaneous knowing which is high in childhood (albeit it is an unconscious knowing), and reaches a nadir in at adolescence.
In modern society adolescence is (spiritually) usually where matters stop - what we call adulthood is not 'maturity' but merely a sustained and degenerate adolescence. Modern 'adults' have lost their spontaneous natural knowledge (instead just passively absorbing propaganda and hypothesies from 'society') and they live in a cut-off state of self-consciousness (so cut-off that it doubts and denies even itself).
The task of adolescence ought-to-be to change that descending line of knowing into an arc - rising in adult maturity to reach the same kind of spontaneous and universal knowing that we began-with - but this time it is conscious knowing.
Thus, children know everything but are unaware of the fact, adolescents know nothing and are aware of the fact; but spiritually-mature grown-ups potentially know everything, and know-what-they-know.
I say adults potentially know everything, because the process of discovering-what-you-already-know is linear and happens in-time; so it would be more accurate to say that knowledge is un-bounded, open-ended, and tends-towards a situation of knowing everything-that-can-be-known - always from the perspective of a single self.
This scenario is, presumably, why all real learning - all knowing of truth - has the distinct feeling of being a realising, a remembering, a recognition... true knowledge is always 'familiar' - we always feel that we 'always knew that' but had never articulated it. I'm saying that we always Did know that - but did not realise we knew it, and could not use that knowing until after we had articulated it.
In terms of knowledge the trajectory is therefore from unconscious knowledge to conscious knowledge; from the implicit to the explicit; from immersion-in knowledge to standing outside it; from passivity through contemplation to creativity.
The idea that we already (in childhood, spontaneously) know everything truly knowable but are unconscious of the fact - and that learning is a kind of remembering and making-explicit and understanding has a complementary aspect. This is that our spontaneous childhood beliefs and wishes have a validity, even and especially when these Bs &Ws have no correspondence with earthly reality, experience or apparent possibility.
An example is flying. I have clear memories of not just yearning to be able to fly (fly by 'levitation', without wings or propulsion - just moving through the air), of knowing what it would be like to fly, and the conviction that it was possible for me to fly... if only I could discover the 'knack'.
More profoundly, I - like most people - was apparently born into this world with the belief that it ought to be a paradise; and that any departure from paradisal conditions was a kind of violation: unjust, against the order of things.
Now, obviously there is no biological basis for human flying, nor any social basis for life as paradise; and therefore such in-built hopes and beliefs are either extraordinary yet common delusions or reality-distortions; or else they relate to a reality that is different from our own, but of which we have memory.
My assumption is that this reality is of pre-mortal spirit life - when we could indeed fly, and life was indeed paradisal. And at an unconscious, implicit, but effective level - we remember this...
We could also, as spirits, do many other things that I believed (against the evidence) was possible; such as read minds, communicate telepathically, change the world by thinking a thing, have my thoughts compelled, move things by a kind of telekinesis, and 'talk' with animals and befriend them.
(Interestingly, such beliefs also re-emerge in people with psychosis and altered states of consciousness.)
In sum, I think that we could reflect more on these childhood, and - in our culture - child-ish, counter-evidential beliefs and desires. And could regard them as destined paths to truth - things we need to become aware of, and to understand.
Note: The above is a version of the 'argument from desire' which was used often by CS Lewis, and also by JRR Tolkien - and which I personally find compelling. I refer to it and provide references in this essay.