I promoted this to a top-level response because I think it's long enough to cover the whole topic. Pardon me for picking this apart in such detail. It's necessary for me to do this in order to understand what's going on.

There are two separate issues here:
Can AS speak untrue words? and
Can AS lie?

We take as a premise that sarcasm is allowed under the oath. So that we're all on the same page here, I will give the definition of sarcasm that I'm using. It is from m-w.com, and is "a sharp and often satirical or ironic utterance designed to cut or give pain." That last part is important. To count as sarcasm, the statement must by definition be intended to hurt the recipient.

Also, I use as the definition of "lie: to make an untrue statement with intent to deceive" which is also from m-w.com

You are claiming three distinct things.
1. The ability to use sarcasm violates the letter of the oath by allowing AS to speak untrue words. As evidence I present your quote


You also claim, 2. The ability to use sarcasm allows AS to lie. I present the title of your original topic, "Cadsuane lies!" and also your statement


Finally, you claim, 3. The use of sarcasm implies the ability to make an untrue statement that is not intended to deceive (so it is not a lie) but is obviously not true.


Your claims are, 3. not true (but, as you said, we both knew that), 2. not true, and 1. true. I think I can tie all these answers together consistently with the practice by the characters of following the oath by adding one concept: the meaning conveyed by a statement.

Your example in claim 3 is exactly what Verin does with Egwene to prove she can lie. She says that Egwene's dress is green. In other words, she makes a statement that conveys no meaning beyond its words, and those words are untrue. So the meaning conveyed is untrue, and it is not allowed.

What about sarcasm? Recall above when I defined "sarcasm," and I emphasized part of the definition by saying, "To count as sarcasm, the statement must by definition be intended to hurt the recipient."

To be sarcastic not only permits one to speak untrue words, it requires one to speak untrue words. By the letter of the oath this practice should not be allowed. But it is. So what does this imply? If we look at the meaning conveyed by sarcasm, it has nothing to do with the words of the statement. The actual meaning of Cadsuane's statement is not "I did not know what you just told me," but is, "You're an idiot" or some such thing. (I'll return to this in my closing)

Now, on to claim 2. Can one use sarcasm to lie? I claim one cannot.

If we accept that a statement is sarcastic, we've already accepted that it was intended to hurt. It cannot, therefore, be also intended to deceive. If a person makes an untrue statement with intent to deceive, that person cannot at the same time be using that statement to ironically make fun of the recipient. It doesn't make sense to intend a person to believe your lie but also intend for them not to believe it and be hurt by it. If you want the recipient to be hurt by a statement (and we know you do because it's a sarcastic statement) they must know the statement is false. So to intend to hurt, you must intend for them to know the statement is false. This is the exact opposite of intending to deceive. So using sarcasm is not the same as lying. The two are incompatible.

To close, I argue that given what we know from studying sarcasm, the intent of a statement does not matter to the oath. What matters is the truth of the actual meaning conveyed by a statement. To lie is to intend to deceive, and is prohibited. To say that a white dress is green does not intend to deceive, but those untrue words are prohibited. Both are prohibited because the meaning conveyed is untrue.

To sarcastically say "I didn't know that" is, in fact, speaking untrue words. Yet it is allowed. I say that is because the actual meaning conveyed, this statement is false and I want it to hurt, is not untrue.

(To anticipate your response, you might say, "Well, it shouldn't be allowed." I don't think that matters, because it very explicitly is allowed.)

Edit:
I've thought about it more, and it seems that the oath will allow a statement to pass if either the wording of the statement or the meaning of the statement or both are true.
So true words + true meaning is obviously allowed.
True words + false meaning, or a lie by omission, is also obviously allowed. We have an overabundance of this in the series.
False words + true meaning is sarcasm (so I claim) and is allowed even though directly contradictory to the wording of the oath.
False words + false meaning is, of course, not allowed.