I see it as taking the Outside View on impossibility. Of course, in real life it usually takes more than a few minutes, but in the Whoniverse it is not unreasonable. Also, asking "How impossible?" seems to me like a good question in some cases.

So long as it is kept in mind that "How impossible?" is merely a more polite and less coherent way of replying "Bullshit. How difficult is it really?".