This would be true if only we were talking about knowing words themselves. The issue here is the definition and the inexplicable, not the word nor prevalency. We can point out those lines on your face because they are there, but this logic breaks down when talking about abstractions. Knowing what the two lines on your face are called and meeting head on with sudden short-term aphasia when trying to describe what "Art" or "God" is are two different scenarios and can't be compared.Here's a good example: what do you call the two lines on your face that come down from your nose to the extremes of your lips?
Well, I was talking about knowing words in the quote. Often enough, we merely don't have the lexicon to describe those, and had we it, we'd describe it just fine. However, I understand what you mean when you talk about what "Art" or "God" is, and I agree with you. Here, it's not what we lack the lexicon - it's that we lack the understanding of the idea, which is why it's impossible to describe. Yet... we understand it, at the same time, do we not? We can grasp that this is art and this isn't in a manner that's inaccessible to us yet working all the time. We understand art, but don't know it. Which means, to me, that there's some sort of idea of art in the background - subconsciousness, if you will - and with right words and ideas, we can probably access it - or, at least, move ourselves closer to understanding it. This is what fascinates me about it.