Don't business plans serve a different purpose then mock-ups? Are you sure you can just switch one with the other and call it the same thing. A mock-up may be contained within a business plan, but a mock-up can't replace a business plan.
The specifics of the product used to be less important. Now with a lot of web and app-based businesses, it's like you can't really explain the opportunity fully without showing the way it works with a mock-up. So that's why they suggest it's a more vital first step to pitching your business idea.