I agree with your general sentiment, but I don't agree that they should necessarily be able to produce a "project plan". In fact, I'd be worried if they had a highly detailed project plan, because in software development, those are rarely kept to (your comparison to the building industry doesn't hold a lot of water in my opinion. It's possible to do deterministic planning in building - what you plan is pretty damn close to what you build. It's very rarely the case that that's true with software).
Still, I'd hope they had some documentation about their project. A well shaped product backlog would be a good start in my books.