Suppose someone asks me what questions you got in the interview. What could I reply if I didn't want to reveal the question?
Is saying: The interviewer asked the question on the call. You wouldn't know what it is.
Is this a rude reply?
I have seen this sentence; I am not sure what the line you would not know what it is means here. Does it mean you won't understand it anyway?
The phrasing in your question taken literally means the interviewer asked you a question, and that you don't think the person you're speaking to knows what that question is. But that's rather obvious given the fact that the person you're speaking to asked you what the question is, so it more implies that they wouldn't know the answer to the question. I agree with your interpretation that the upshot is to imply the person you're speaking to wouldn't understand or be able to answer the question anyway, which could be interpreted as rude - you're making a judgement about the other person's (lack of) knowledge or interview skills.
If you don't want to share the question you were asked, just say so. There is no need to attribute your reluctance to share the question to the other person's perceived inability to understand or answer the question.
This has really nothing to do with English, but with culture. There is a very easy way: You tell him “These are interview questions. The company wouldn’t want them to get out and give someone an unfair advantage”.
Whenever you come up with fake excuses, you run into problems later on.
“You would not know what that is” in the English language is indeed quite rude, rude enough to be remembered and paid back to you with interest. It’s a phrase that you should avoid, even if it’s true - unless of course being rude is what you wanted.
