What does screen-count have to do with simplicity?
You can clearly define 6 major features all to support a single goal: facilitating traceable, text-driven professional communication (my personal impression).
You have some clear guidelines to make the experience consistent along several dimensions.
All this makes that Basecamp to me is a simple tool (as mentioned on the about page), or better a carefully assembled tool set if part of them are actually also separate products.
But I guess all this does is ask for a good definition of simplicity. One that doesn’t equate it to small or easy. One that doesn’t put it as the prime goal, but as a means to meet other goals or a consequence of wanting to reach thes goals.