Your manager cares about the product, he's emotionally invested in it and wants everything to go as fast as possible - it's not a bad thing!If he was emotionally invested in the product, wanting everything to go as fast as possible is not necessarily a good thing. I can make things go as fast as possible, but then the questions become no unit tests? no integration tests? spaghetti code?
And this isn't a response from a few go around either. This is often the first thing that comes out of their mouth, right at the beginning of the project.
What happens is as follows:
1) Your manager has software development background and uses it as a superiority contest to show how much better they are than you. His lack of awareness regarding variable skill level or lack of knowledge of an existing system seems to factor little into the question. So one could argue the manager could be more empathetic in this case.
2) Your manager knows nothing about software development and really has no business asking such questions in a condescending manner. So they either pretend to know or they promised someone higher up the chain even though they had no idea, but were scarred to say so, thus the cycle has completed.
3) Your manager has a calm, rational head and accepts your estimate, your explanations, and has a good understanding of the product to negotiate with you in terms of requirements without getting emotional. The manager trusts understands that your estimate is, more than likely, not going to be accurate but accepts the changing winds and doesn't hold you to the fire.
I totally agree its better to be explicit. Which is why it matters to avoid overly emotional reactions and accept what people tell you so hand waving can be avoided.
But this is more of a prevailing problem in software requirements in general. It's rare that developers get projects with clear ideas on what needs to be done because of the ignorance or unknowns in questions that need to be asked in order to deliver a product. Often the developer has to decide, and when they do and its wrong, then they have to fix whatever they decided because no one explained, or bothered to explain, how something was supposed to work in the first place.