The top down approach requires you to have deep domain knowledge to build a successful product around your idea. The bottom up (lean) approach starts the same way but is based on measurable feedback. In both approaches, you still start with the idea. Domain knowledge MAY improve your odds of creating a good product but plenty of people have done/tried this and failed miserably. The lean approach improves your odds of success by actually trying to measure success whenever possible and use those measurements to validate or improve the process. The days of "if you build it, they will come" are numbered in the face of more data driven methodologies.