For the sake of simplicity, I am going to write my response as if you were the engineer in question.
Usually, you ask these kinds of questions before you join a startup, not after. And yes, the type of investors and the size of the investment can affect your decision to join such a startup in the first place.
"You will never know that. Why do you want to know that? The funding is legitimate. That's all you need to know."
Consider yourself warned.
You are working for a jerk. I'm not saying this person was wrong to keep that kind of information confidential. He wasn't necessarily. But he definitely could have been a lot more diplomatic in his response. There are ways you can say 'no' to someone without making them feel like crap for asking the question in the first place.
For all he knew, you were just curious, or maybe you wanted to help with the fundraising efforts. And even if you were not interested in helping with raising funds, being curious about the strategic direction and potential longevity of your company shouldn't be considered a sin.
In fledgeling startups where tech is a key component of the business model, full time engineers are often included in such high level discussions. For many, this inclusion is what keeps them working for the new startup over the long term, despite the fact that over time they could probably get a higher paying job at a larger and more stable company elsewhere.