If the company declares it confidential, leave. They don’t have a plan.
This can't be true for all startups. Some ideas are worth being confidential. Yeah I know execution matters most, but still idea can be what they are excited about.
You are technically correct, but in this case it's not the best kind of correct. Of course it's not 100% for sure they don't have a viable plan, but it's likely they don't and more importantly it's not worth your time to find out. The OP is right: leave.
If their business plan is so simple that they will compromise it by telling you, they don't have a business plan. If they can't identify the small bit of secret sauce (algorithm or research, unique understanding of/access to the market, unique set of competences etc) that will make them successful and thus needs protection, they either don't have it, or they don't understand it. Either case is bad.
This can't be true for all startups. Some ideas are worth being confidential. Yeah I know execution matters most, but still idea can be what they are excited about.