How did you arrive to your suggested interpretation?
The jargon of "knowledge by acquaintance" and "knowledge by description" comes from Bertrand Russell. Knowledge by acquaintance is "direct" or "experiential" knowledge, such as knowledge of a pain or other sensation that you're having. Knowledge by description is second-hand knowledge obtained by processing a proposition, e.g. your knowledge of my pain on the basis of what I tell you about it.
What I was picking up on in Tuukka's statement was that the irrationals are uncountable whereas the rationals are countable. So the rationals have the cardinality of a set of discrete combinatorial structures, like possible sentences in a language, whereas the irrationals have the cardinality of a true continuum, like a set of possible experiences, if you imagined qualia to be genuinely real-valued properties and e.g. the visual field to be a manifold in the topological sense. It would be a way of saying "descriptions are countable in number, experiences are uncountable".
I've seen there's discussion on LW about rationality, namely, about what it means. I don't think a satisfactory answer can be found without defining what rationality is not. And this seems to be a problem. As far as I know, rationality on LW does not include systematic methods for categorizing and analyzing irrational things. Instead, the discussion seems to draw a circle around rationality. Everyone on LW is excepted to be inside this circle - think of it as a set in a Venn diagram. On the border of the circle there is a sign saying: "Here be dragons". And beyond the circle there is irrationality.
How can we differentiate the irrational from the rational, if we do not know what the irrational is?
But how can we approach the irrational, if we want to be rational?
It seems to me there is no way to give a satisfactory account of rationality from within rationality itself. If we presuppose rationality is the only way to attain justification, and then try to find justification for rationalism (the doctrine according to which we should strive for rationality), we are simply making a circular argument. We already presupposed rationalism before trying to find justification for doing so.
Therefore it seems to me we ought to make a metatheory of rationality in order to find out what is rational and what is irrational. The metatheory itself has to be as rational as possible. That would include having an analytically defined structure, which permits us to at least examine whether the metatheory is logically consistent or inconsistent. This would also allow us to also examine whether the metatheory is mathematically elegant, or whether the same thing could be expressed in a simpler form. The metatheory should also correspond with our actual observations so that we could figure out whether it contradicts empirical findings or not.
How much interest is there for such a metatheory?