It is a question, as the verb is in the first position of the sentence. It does mean; is a statement. Unless I am misunderstanding your question.

I would need more context to be sure but I don't think "That's not a question" means "No doubt about it" in any situation that I can think of. "There is no question about it", "There is no question", or just "No question" can mean "no doubt" or one might say "that's not THE question" if somebody asks a question that's irrelevant or the answer is obvious" I think "That's not a question" is more likely to be used when somebody makes a polite request and then changes it to an order when the request is not carried out immediately. For example, a chef might ask his assistant "Could you get me some water?" and then when the assistant is busy and doesn't get the water right away, the chef becomes impatient and says "That's not a question" to tell the assistant that he wants him to get the water immediately.

It can mean "No doubt about it", regionally we have some interesting expressions. I apologize as I misread your question. It would depend more on the context of the conversation, in my opinion.