Not sure I agree with that order. I personally know people who have learned SQL before php or other backend language. You make a good point about the other elements in website building. It can be invaluable to have a good designer/user interface expert type on your team. In my experience _most_ hacker types are weaker in those areas, myself included.
It should also be noted that building a website is also an exercise in graphic design, writing, and user interaction design. Anyone of those other facets can be equally difficult to learn and master.