But what's the point of asking for no answer
What do you mean? He called Flynn at 3am because he's an idiot, thinking "people are asleep at this time, it's secret hour!". Flynn was caught off guard, like "How stupid is this guy calling me FROM THE WHITE HOUSE AT 3AM TO ASK ME ABOUT THIS?" he's the NSA and not supposed to give him advice on that. So Flynn just acts coy and says he doesn't know and should ask an economist. Trump was reportedly a bit upset at Flynn not giving him an answer, as if it was normal for Flynn to usually give him advice on any topic and he suddenly didn't understand why Flynn was telling him to ask someone else all of a sudden, maybe even on a topic they had already talked about before.
There is no reason at all that we should know about this conversation, only Trump and Flynn would know about it, if it's true, unless Flynn then told other people. Why would he tell anyone "Hey Trump called me last night at 3am to ask if we need a strong or weak dollar, lol, I told him to ask an advisor! Silly guy!"?
Let's take an example. Assume there is suspicion that a coach and a ref have been colluding in hockey matches to rig things up, a criminal investigation is going on, and one day the coach calls the referee at 3am:
Coach: "Hey bud, who should I put on the first line tomorrow? I forgot."
Ref: "Err I don't know, I'm just a referee, you should consult your assistants."
Coach: "Uh? Why? Why won't you tell me? I'm just wondering, Deharnais or Gallagher? I don't remember."
Ref: "Goodnight sir."
Coach: "What? Hey bud help me out here."
*click*
Next morning, the ref shows up at the arena, and as they prepare for the game he goes around telling people:
Ref: "Hey buds, last night, the Hab's coach called me to ask who he should put on his first line. Told him to consult his team you know, who am I to tell him? ha! Crazy ol Habs coach right?"
Why? Because he now makes a conversation that should be kept secret public, because doing otherwise would just reinforce the fact that he kept this very strange conversation secret. He passes it off as the coach being dumb and nothing more. Keeping it secret until it's eventually revealed would put him in trouble, a prosecutor would grill him on it. "Do you think it's normal? Why didn't you tell anyone then?".
Sure maybe Trump is just really fucking stupid and Flynn told people, making Trump look stupid, but considering their relationship I doubt it. I think Trump consulted with Flynn on any policies before he got elected, and we can imagine how Flynn figured what policies Trump should follow. Trump was just too stupid to stop doing so straight up from the White House's phone line once he got in, and called him at 3am to ask him about dollar policy. "What? He's my advisor guy, I can ask him this stuff."