What people might be missing is that you don't do anything "lean" without refactoring sprints. So as you grow, you keep considering the macro structure. If not, you should use TDD at least.

Nevertheless, validation is key, from an executive POV. It makes sense that things are dirty in the beginning, but they should start being encapsulated, object oriented, properly interfaced, etc. as soon as you feel in need for a second programmer.