Can incredulous mean incredible?
incredulous as incredible was revived in the 20th century after a couple of centuries of disuse. Although it is a sense with good literary precedentamong others Shakespeare used itmany people think it is a result of confusion with incredible, which is still the usual word in this sense.
Oxford is dictating language in this case my dear Draignean.
That and your definition isn't correct even if we ignore this. Incredulous can also refer to an action or mannerism that implies disbelief.
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https://en.oxforddictionaries.com/definition/incredulous(of a person or their manner) unwilling or unable to believe something.
Neon. How did you read that definition and somehow take away that "That is incredulous!" is a meaningful sentence? You could say "That is incredulity!" if you want to point out someone who was acting in an incredulous manner. The sentence as you have it is syntactically sound, but semantically gibberish.
I'm serious here, at this point we're both looking at the same dictionary, and I'm utterly dumbfounded as to why you are (yet again) attempting to use words in a way they were never meant to be used.
If you want a bit more proof, check this out:
google ngram viewer. No literature on google has ever used that phrase to end a sentence. You can check the period form too. It doesn't exist.
Seriously, Neon, can you find any example of any other human being using that phrase as part of normal literature? I don't want to drag ameripol into a grammar debate, but seriously, it is not a bad thing to go: "Oh, right, I meant X" and then move on.